Bill Potts

Bill Potts (born circa 1991) was the final companion of the Twelfth Doctor. She shared a teacher-student relationship with him similar to that of Leela and Gabby Gonzalez, albeit on more official circumstances.

When she first encountered the Doctor, Bill was working as a canteen assistant at St Luke's University, seemingly destined for an unprivileged life. When free from work, she attended some of the Doctor's lectures at the university. The Doctor saw her unfulfilled potential and invited her for a series of personal tutoring sessions.

Following an encounter with a time-travelling puddle that was piloted by her new crush, Heather, Bill discovered who the Doctor really was — an alien time-traveller, guarding a secret vault beneath the university campus. The Doctor later invited her to join him on a journey through space and time.

On her adventures with the Doctor and Nardole, Bill travelled to a human colony planet and visited the last of the great frost fairs. She faced the vacuum of space and saved the Earth using the memory of her mother. She also learned that the Doctor was trying to rehabilitate his old Time Lady friend. Bill also helped restore the Doctor's lost eyesight in exchange for temporarily helping aliens take over the Earth.

When accompanying the Doctor on his test to see if Missy could be good, she was fatally shot on a Mondasian ship by Jorj. She was then converted into a Cyberman against her will; though she retained her sense of self, thanks to her strong will.

After helping the Doctor destroy the Cybermen, Bill was rescued from her fate by Heather, who had returned for her and converted Bill into a sentient oil being like herself. After setting the TARDIS to take the Doctor away, Bill left with Heather to explore the universe.

Before the Doctor
Bill was born around 1991. (TV: The Pyramid at the End of the World) Her mother died when she was young, so she was brought up by a foster mother called Moira. (TV: The Pilot) Bill did not openly express her sexuality, preferring to only reveal it when required. Her adoptive mother was oblivious to her being a lesbian, assuming that Bill was interested in men. (TV: The Pilot, Extremis) As Moira was never able to hold down a single partner, Bill tended to never bother learning their names as "[she'd] only get attached." (TV: The Pilot)

At St Luke's University
Later in life, Bill served chips at St Luke's University in Bristol, and — despite not being a student — attended some of the Twelfth Doctor's lectures. Noticing her potential, he became her personal tutor. Bill also accidentally fattened up a girl she fancied by giving her extra chips; though she lost interest in the girl, Bill continued to give her extra chips as a gesture of kindness.

During Christmas 2016, Bill mentioned to the Doctor that she was "supposed to look" like her mother, but, as her mum rarely had any pictures taken, Bill didn't really know if she looked like her. When she went to the flat where she lived with her mother, Moira told her she had found a box with pictures of Bill's mum. On one of the photos, Bill could see the Doctor's reflection.

Over the course of her tutoring in 2017, Bill developed a crush on another student, Heather, who showed her a puddle that cast a reversed reflection. Bill asked Heather not to leave without her. As a result, when Heather was taken over by the puddle, she followed Bill home, so as not to leave without her.

Bill went to the Doctor's office, and then he brought her into the TARDIS. He tried to outrun the water creature, and finally travelled to a war zone in the Dalek-Movellan War. Bill realised she was being pursued as Heather had promised not to leave without her. On the Doctor's advice, she released Heather from her promise.

The Doctor took her back to the university, where Bill expressed regret at having turned down Heather's offer, with the Doctor telling her it was unlikely they would ever see each other again. When Bill knowingly looked at the TARDIS, the Doctor realised what she was suggesting and made her stand close to him, intending to wipe her memory of these events, but she convinced him not to. He then agreed to take Bill with him on his travels, acknowledging that they might run into Heather one day. (TV: The Pilot)

Travels with the Doctor
The Doctor took Bill to visit a human colony in the far future, telling Nardole that he was just transporting the TARDIS back to the office. When the Doctor discovered the Vardy robots had killed the set-up team on the colony, the pair almost blew up the ship around which the city had been built before realising that the colonists had already arrived and were in hibernation. The Doctor stopped the bomb, and the pair deduced that the Vardies had been built to sustain happiness and had come to view grief as a plague.

The Doctor rebooted the Vardies, erasing their memories of the humans, before beginning diplomatic negotiations between the humans and Vardies. He then tried to return to the university just after they left, but when he and Bill exited the TARDIS, they discovered they were outside in winter with an elephant in front of them. (TV: Smile)

On exploring London in 1814, the Doctor and Bill attended the last Frost Fair on the River Thames before noticing strange lights under the ice. Bill witnessed firsthand the death of a little boy when he was sucked under the ice and was horrified and disgusted when the Doctor seemed to care more about retrieving his sonic screwdriver than the boy who had been stolen. Bill challenged the Doctor demanding to know whether he had ever killed anyone to which he confirmed saying that he did not have the luxury for outrage as he was 2000 years old. Bill befriended the urchins who had originally stolen from them and chose to cooperate with the Doctor. Their investigation led them to Lord Sutcliffe with the Doctor warning Bill to let him do the talking as it required a diplomatic approach. After Sutcliffe made racist remarks towards Bill, however, the Doctor punched him in the face. Later Bill agreed with the Doctor to set the creature in the Thames free, surprised when it wasn't headline news in the present day. (TV: Thin Ice) The Doctor took Bill to Titan to see a clear image of Saturn, her favourite planet. He then introduced her to the concept of "swimming" in the gravity-deprived atmosphere, although this was cut short when the area around them was heavily disturbed by a passing land ship. When the pair were rescued by the passing vehicle, Bill was introduced to Rudy Zoom, the financier of the voyage. The two soon met Lady Takashi, a psychic who felt an alien presence on Titan years ago in a sleep. The ship changed its buoyancy to float on the methane river. After several hours, they found a domed-off garden in a cavern, which they investigated on foot. The Doctor noticed that all of the vegetation inside was sentient, except for two trees in the centre. The group was scattered when a robot attacked them, believing them to be weeds. As Bill hid in the garden, she was attacked by one of the Haluu plants and passed out. Bill found herself in the Dreamspace where Sythorr — who was the giant Haluu below the two trees — chased Bill and Rudy in an attempt to wipe their minds as part of his plan to take over Earth. They escaped and projected themselves into the real world until they woke up. Sythorr's lover Oksanna ordered Bill and Rudy to be released. As they ran for the doors, Oksanna sacrificed her life to destroy Sythorr and the domed garden. After making it to the ship, Bill saw the last piece of Sythorr warn the Doctor before dying that "The unknown soldier...is stirring." (COMIC: The Soul Garden)

The Doctor and Bill landed in the Indian Territory in 1880, where the outlaw Seth Shelton tried to hold Bill hostage. She was saved by United States Deputy Marshal Bass Reeves and the scout Joey Two Trees, who had Shelton arrested and intended to take him back to Arkansas to stand trial. While Reeves, Joey and the Doctor took off, Seth's brother Frank Shelton arrived at Reeves' wagon and freed Seth. Seth and Frank waited for the Doctor and his friends to return so they could kill them. The Seminole medicine woman Totika summoned the Stikini who killed the Sheltons by ripping their hearts from their bodies. The Stikini threatened the life of Zeke Tolbert, but Bill promised to tell Totika the truth about Bill's travels to other worlds. Totika then put Bill to the test to swap her body with Stikini War-Queen Cocheta in the Red Skies, a region of the Dreamspace the Stikini were native to. The Doctor and Reeves freed Bill during the ceremony, but the ceremony succeeded. In the Red Skies, Bill met the other Seminole Totika had banished. The Doctor contacted Bill, telling her where she was. Bill and the Seminole willed themselves out of the Red Skies, forcing the Stikini back there. Reeves and Joey destroyed the totem used in Totika's ceremonies. Afterwards, the sky opened up, and the group saw the Stikini get burnt by an unknown force in the Red Skies. Returning to the TARDIS, the Doctor and Bill saw the TARDIS had impossible carvings on its exterior, and the Doctor demanded answers. (COMIC: The Parliament of Fear)

Searching for answers about the carving on the TARDIS, the Doctor came to the Renath Archive library on Cornucopia to try to identify it. Angered by the head librarian Matildus Galathea's snobiness, Bill went outside to the Kaballus Quarter for some fresh air. Noticing Bill was the first to come in or out of the library in months, the Kaballus Kids captured Bill, and told her that Matildus used to let them hang around before she behaved strangely and barred the kids. Bill showed them a photo of Matildus being grumpy, which revealed something strange around her head.

Returning to the library with the Kaballus Kids, Bill told the Doctor, Matildus and Sashana to stop whatever it was they were doing. Bill took a photograph on her phone which revealed Sashana's true form. Sashana revealed that she was psychically sowing doubt in Matildus' mind and blocking her memories so Sashana could sell off the library's collection after being handed custody of it. She began to fry the group's brains with her mind, but was stopped after Bill summoned the Ristallian crater-hound Archie, whose simple brain couldn't be kept back. After Sashana was arrested, the Doctor realised the carving wasn't a symbol, but a coded message. The Doctor and Bill's next destination to break the code was to see Alan Turing. (COMIC: Matildus)

The Doctor and Bill went to visit the Galatean duplicate of Alan Turing on the lunar colony Athenia to decipher the markings on the TARDIS. However, Alan had vanished two years previously. The Galatean Chiyoko, feeling her "mother" (the TARDIS's essence on Bill), had Bill take Chiyoko to the TARDIS, which Chiyoko could hear was screaming due to the markings on the TARDIS' shell. After reuniting with the Doctor, Chiyoko told him where to find Alan. They found Alan inside a replica of King's College, Cambridge on the Moon's surface. The arrival of the TARDIS caused the code, which was part of the Dreamspace-dwelling Phantom Piper, to make the Piper manifest through Alan using Block Transfer Computation. When the Phantom Piper used Chiyoko's mind to create soldiers from every war to start a war between humans and Galateans. Bill told Maggie Cortez not to kill Chiyoko, and told Maggie to trust the Doctor. With the help of a suggestion from the Doctor, Alan defeated the Piper by breaking down his code, allowing Chiyoko to live. (COMIC: The Phantom Piper)

Shortly afterwards, the Doctor and Bill ended up stranded in an entropy bubble in space, as part of an elaborate trap by Fey Truscott-Sade to capture time travellers, including the Doctor. During an attack by the Clockwise Men, Bill helped Gol Clutha attack them with a proton cannon, but was powerless to stop them ageing Gol to death. Escaping, the Doctor and Bill landed back on Cornucopia, where Bill shouted at him for not telling her enough about what was going on. After apologising and meeting Annabel Lake, Bill narrowly avoided being shot at by Fey during their flight to Wonderland HQ, leaving her shaken for being targeted instead of the Doctor.

After the Doctor explained to Bill his past with Fey during the Time War, they met with Matildus, where Bill watched over the Doctor as he fell asleep to meet Fey in the Dreamspace. Later, when it was discovered that Fey's only surviving relative, Alexander Truscott, was probably behind things, Bill recognised him from his appearances on TV. After she and the Doctor got captured in an attempt to storm Truscott's house, they eventually escaped, where Bill first noticed the globes from London's CCTV cameras floating and forming around Fey, preparing to attack. The globes reconstituted as Shayde around Fey, and Bill joined the Doctor's group in entering the Dreamspace inside. After Fey attacked the Doctor, Bill finally got her to listen by explaining how the Doctor always tries to do what is right, allowing the Doctor to reveal the truth to her: that the Absence was never real. After the revived Shayde wiped Fey's memory of her time with him at the cost of his own life, Bill joined the Doctor in placing her safely in Wonderland in London, although she was upset that the Doctor never said goodbye to her. (COMIC: The Clockwise War)

She later helped investigate and stop the Shining Men, (PROSE: The Shining Man) and visited Kollo-Zarnista Mining Facility 27 above Saturn. (PROSE: Diamond Dogs)

Bill and five other university students were looking for accommodation when the Landlord invited them to stay in 11 Cardinal Road. The Doctor also investigated and discovered the Landlord was feeding students to the Dryads, insects who were working with the Landlord to keep his mother Eliza alive. Bill led her own investigation and discovered the secret of the house. After the Landlord was defeated, the house disintegrated. (TV: Knock Knock) She later moved back in with her foster mother. (TV: Extremis)

Travels with the Doctor and Nardole
Bill visited the mining station named Chasm Forge after the TARDIS received a distress call from it. As oxygen was regulated and the TARDIS exterior air shell was being removed, Bill had to wear a smartsuit that provided her with a force field which held air in. When entering an airlock which required a solid physical helmet, Bill's malfunctioning suit automatically took her helmet off. This meant Bill was exposed to the vacuum of space. The Doctor later gave her his helmet, at the cost of his sight. (TV: Oxygen)

After viewing an email from his virtual counterpart, the Doctor called Bill asking if she knew a person called Penny. Bill did, but deemed her "out of [her] league". The Doctor assured her she wasn't and told her to call Penny that night as he had a "feeling [they were] going to be very busy". (TV: Extremis)

Bill went on a date with Penny and told her about the virtual world the Doctor had since told her about. Penny was disbelieving, but the date was going well until armed soldiers and the Secretary-General of the UN arrived to collect Bill and take her to see a pyramid that had appeared overnight. On board the Doctor's President plane, Bill tried to convince the Doctor in the TARDIS to come and help with the pyramid - but he denied until he realised he didn't have a choice as the TARDIS had already been loaded onto the plane.

At the pyramid, Bill was confused when Nardole was 'updating' the Doctor on what was happening in front of them as the latter approached the pyramid. Her phone, like every other device with a clock, received a message saying 23:57 pm as the Monks started a countdown to the end of the Earth, demanding surrender. The Doctor, Bill and Nardole then took the Heads of the American, Russian and Chinese armies and had them strike the pyramid in a coordinated attack.

The Monks stopped the attacks with ease and expressed they were ready to talk. Inside, the Monks revealed that Earth would be dead in a year's time if the planet did not consent to the Monks' help.

The Doctor was against consenting to something he didn't understand, though Bill and the others were more willing. However, the Monks explained that love was needed in consent, burning the Secretary-General when he consented out of fear.

Back on board the President plane, the army heads agreed not to fight one another believing that they would be stopping World War III, but this failed to revert the clocks. Bill compared the need for consent as to inviting vampires into a person's home. She also theorised a bacterial possibility and the Doctor had them all Google possible problems. However, Bill and the army heads were considering surrendering Earth to ensure the planet survived, unwilling to put such risk especially when the clocks moved to 23:58 pm.

As the Doctor and Nardole deduced the problem as a biological component released in a lab, Bill and the army leaders went to the Pyramid as the countdown reached 23:59pm. Inside, the army leaders were burned by the Monks for consenting out of strategy. Bill went to leave when the Doctor managed to solve the biological problem and the countdown began to go backwards. However, when the Doctor could not leave due to being blind and unable to spin the combination lock, Bill returned and consented to the Monks having authority over the Doctor in return for his sight being restored. Before the Monks accepted her 'pure' consent out of love, she told the Doctor to save her planet. (TV: The Pyramid at the End of the World)

Bill was reunited with Nardole during the Monks' reign over Earth. She had been away from Nardole and the Doctor for six months since their last adventure. The Monks had inserted themselves into history during this time, however, Bill had refused to fall for the mental deception.

Nardole located Bill and helped her find the prison hulk where he believed the Doctor was making his broadcasts and was captive there. The Doctor told Bill he was cooperating willingly with the Monks, believing that humanity was doomed without their guidance. Bill became distraught and shot the Doctor with a gun she took from one of the SWAT troops, who appeared to begin regenerating, but quickly stopped. The entire scenario was revealed to have been a test by Nardole, the Doctor and the team, to make sure Bill was not under the influence of the Monks.

At the university where the Doctor and Bill worked, they entered the Vault to talk to, who said she had encountered the Monks before. She confirmed the Monks maintained control by broadcasting a signal containing the false history to their subjugated victims via the numerous statues they had built across Earth, enabled by a psychic link established through the person who originally gave "consent"; Missy claimed to have defeated them during her own encounter by killing that individual. She implied that Bill must die as the one who gave consent on Earth.

Bill accepted her fate, but the Doctor was determined for another solution. Along with Nardole and the SWAT troops, they infiltrated the Monks' pyramid in London so that the Doctor could hijack their broadcast with his own mind to break the psychic transmission. In the central chamber, the Doctor attempted to link his mind to the controlling Monk, who after a struggle overpowered the Doctor. Bill intended to sacrifice herself, linking her own mind to the Monk's despite the Doctor's protests. The Monks' broadcast was replaced with images of Bill's mother, which the Doctor recognised as a strong and loving memory in Bill's mind. Humanity woke up from the Monks' lies and revolted against them. The Doctor, Bill, and Nardole watched as the Monks abandoned Earth. Sometime later, the Doctor and Bill found that most of humanity had no recollection of the Monks. (TV: The Lie of the Land)

With Nardole joining the Doctor and Bill on off-world adventures, the trio visited NASA and discovered the phrase "God Save the Queen" on the surface of Mars. The trio arrived on Mars in 1881 and Bill became separated from the Doctor when she fell down a hole, causing Nardole to return to the TARDIS and get stuck there when the TARDIS returned to Earth. The Doctor and Bill reunited and discovered an Ice Warrior called Friday working with a group of Victorian soldiers. After the Empress was awoken, Bill and the Doctor were locked up by Captain Catchlove, who wanted Mars for himself. Friday rescued them and Bill attempted to prevent an all-out war between the Ice Warriors and the humans. When the crisis was resolved, the TARDIS returned and Bill learnt that Missy had been let out of the Vault by Nardole to rescue them. (TV: Empress of Mars)

Bill, the Doctor and Nardole travelled to Edinburgh in 1645, where they encountered a city plagued by Ghosts and the actual Plague. (PROSE: Plague City)

After Bill got into an argument with the Doctor about the fate of the Ninth Roman Legion, they, along with a reluctant Nardole, travelled to 2nd century Scotland. Bill once again got separated from the Doctor when she was chased by Kar and then encountered a Roman. She witnessed the Roman get eaten by a mysterious creature and got rescued by the remaining Romans, who survived the creature's attack by running away. After being knocked unconscious for two days by the creature's slime, Bill awoke and worked with the Romans to find the Doctor. Upon being attacked by the creature again, Bill and the Romans escaped and encountered the Doctor and Nardole with the Picts. Bill and the Doctor helped the two sides work together and then worked to send the creature back through the portal where it came from. When the Doctor attempted to stay behind and guard the gate, Bill knocked him to the ground to stop him and reminded him of his duties to guard the vault on Earth. When they returned to the TARDIS, Bill and Nardole discovered that the Doctor had let Missy out of the vault for "maintenance". Despite his two companions not trusting her, the Doctor maintained the belief that Missy could change. (TV: The Eaters of Light)

Bill and Nardole, watched over by the Doctor, became Missy's companions on a test trip to determine whether she could reform from her evil ways. While surveying the colony ship they were on, Bill was mockingly called the "exposition" by Missy. Bill was mortally wounded by a paranoid technician who was the only one left of his crew, due to her being human and supposedly a target for an unknown threat. Unable to save her, the Doctor allowed her to be taken by medical figures who had come from the lower levels of the ship as he understood were capable of healing her. He left a message in her subconscious for the moment she awoke: "wait for me". (TV: World Enough and Time)

Cyber-conversion
After several months for her, she awoke with a cybernetic heart in a hospital. Due to the spaceship's proximity to a black hole (causing time dilation within it), Bill was forced to wait a total of ten years for the Twelfth Doctor and in the meantime made friends with a man named. Razor allowed her to watch the Doctor and the others upstairs, though time passed extremely slowly; by the time the Doctor raised an eyebrow, a week passed for Bill. Razor also took her outside the hospital once, explaining how everyone had to be converted to survive given the time differential with the upper levels. Bill, wanting to be with the Doctor, began to repeatedly ask him to take her to the lifts. Eventually, Razor apparently caved in and seemingly took her to the lifts. However, he had actually taken her to the Conversion Theatre, telling her that it was for her own good. She was then forcibly converted into the first Cyberman. Sometime later, the Doctor, Nardole, and arrived. In the theatre, the Doctor found her, and asked her if she knew where Bill was, and told him that she was Bill, telling him, "I waited for you," as Missy, and Razor, (really the Master) took their places at her sides. (TV: World Enough and Time)

Bill managed to reject the Cyber programming enough to help save the Doctor from another Cyberman, destroying it, and carry him to safety after the ship Nardole piloted crashed on Floor 0507, a solar farm. While staying there, she still believed she was herself, unable to understand why the villagers were scared of her. When Alit gave her a mirror, Bill realised she was in Cyber-form, causing her distress. An injured Doctor arrived and explained to her what a Cyberman was and how she was converted, speculating her mind now acted as a perception filter so she would still believe she was human. Bill, upon seeing her shadow, which confirmed she was a Cyberman, grew angry at the Doctor for not saving her, accidentally blasting a hole in the barn that further startled the villagers as they prepared for war. The Master soon arrived, informing the pair that his future self had found something of interest, momentarily teasing Bill about how he hated the ten years he had to spend with her on Floor 1056. Bill pretended that his cruelty did not upset her but was crying beneath her mask, something which surprised the Doctor.

While following the Master, Bill asked why the Cybermen were planning to invade Floor 0507, the Doctor replying that it was due to their desire to take the children. Bill questioned why the children specifically, though the Doctor withheld his reply, causing the Master to finish for him: "There is less to throw away". As the Doctor stumbled against a nearby tree, Bill began to lose hope, believing the scenario they found themselves in was inescapable- the Doctor even admitting he didn't know how he would get Bill back to normal. However, he briefly brightened Bill up by saying there was hope because she was still able to produce tears.

They arrived in the middle of a forest to find Missy using her sonic umbrella on a camouflaged lift. She briefly empathised with Bill's condition before calling the lift. Understanding a Cyberman would be in it, Bill ordered the others aside to combat the threat directly, the three Time Lords preparing their respective sonic devices behind her. An advanced model of Cyberman came from the lift, but the four managed to disable it after several seconds.

Bill later created a passageway for the children to escape in. When she went to inform Nardole of this, Hazran fired at her, believing her to be a Cyberman. This upset Bill and she went to talk to the Doctor, but before long the Cybermen arrived. The Doctor and Bill chose to remain and fight off the Cybermen despite Nardole's protests. He later bade farewell to Bill and the Doctor and was sent to defend the children.

In the final battle, she was allowed to join the Doctor as he confronted the Cybermen. After the Doctor blew up Floor 0507, Bill was left alone on a battlefield. Heather appeared from a puddle and Bill found herself back in her body, and saw her Cyberman shell fall to the ground, dead. After taking the Doctor back to the TARDIS, Heather explained that she traced Bill through the tears she left; and also that Bill was now like her, a Sentient oil creature, but she could be made human again if she wanted. Once she had left her tears with the Doctor, Bill went to travel the universe with Heather. (TV: The Doctor Falls)

Later life
Bill and Heather travelled the universe together and visited all the places the Doctor never got around to showing Bill. Eventually, Bill decided she wanted to try being human again, if Heather was willing to live a human life on Earth with her. They purchased a home together, went clubbing, and grew old together. (PROSE: Twice Upon a Time)

Death
While dying of old age, Bill gave permission to Heather to go back out and continue travelling the universe. (PROSE: Twice Upon a Time)

Legacy
Bill's memories were retained by the Testimony Foundation, which assumed her form as it interacted with the Doctor as well as his first incarnation and Archibald Hamish Lethbridge-Stewart. Though the Doctor, who believed Bill to have died in the Battle of Floor 0507, was sceptical as to the authenticity of the Bill's image, she repeatedly insisted that she was truly Bill Potts. (TV: Twice Upon a Time)

Personality
Bill was very smart and inquisitive. She asked questions that most people didn’t ask, and admitted that "Questions [were] kind of [her] thing", (TV: Twice Upon a Time) and came to conclusions in a different way than other people normally did. She loved science fiction and was open to the idea that the concepts it proposed were real and affected the lives of those around her. When Heather introduced her to a strange puddle, she was open to the idea of alien lizard possession, no matter how preposterous the Twelfth Doctor found it. She was also easily able to figure out that the Doctor intended to wipe her memory when he tried to put his hands on her head, and even called out the Doctor for thinking she was stupid enough to fall for it. (TV: The Pilot)

Bill's relationship with Moira tended to be similar to roommates rather than mother and daughter. Backing this up is that Bill tried to keep out of Moira's personal life by never remembering her partner's names; Bill hinted she actually got along well with some of them. (TV: The Pilot)

Bill was also rather bold, hating the idea of being spoken to in a sexist manner and having to withhold her opinion due to racial stereotypes of the 19th century. When Lord Sutcliffe demanded she submit to him, Bill merely sneered at him. She also learned to understand the need to not dwell on matters that angered her as the Doctor taught her there simply wasn't time to waste on such matters. (TV: Thin Ice)

Bill was also more prone to profanity than many of the Doctor's other companions. She was implied to have sworn on multiple occasions, (TV: Thin Ice, The Lie of the Land) and also once called the Doctor a "bloody arse". (TV: Twice Upon a Time)

Bill was a lesbian. (TV: Knock Knock) She had a crush on a girl who attended the Doctor's lectures, and developed an interest in another young woman, Heather. (TV: The Pilot)

Despite having never met her mother, Bill invented an imaginary version of her which she spoke to for personal comfort. (TV: Oxygen, The Lie of the Land) According to Bill, this version always told her "with some people you can smell the wind in their clothes", a saying Bill made up based on what she thought her mother would say. (TV: The Pilot)

Skills
Bill was extremely bright and quick-witted, able to perform well in the Doctor's university assessments with his private tuition in the field of science, getting a First in every assignment, and impressed him enough beforehand that he saw potential in her. She was also able to deduce complex puzzles with only some prompting from the Doctor, notably that the puddle in the ground was not reflecting as a mirror and that the damaged cement was actually scorch marks of the ships, (TV: The Pilot) and that the lights, which she perceptively noticed, under the ice in the Thames were organic rather than artificial. (TV: Thin Ice) Bill also deduced the TARDIS granted her the ability to speak different languages, (TV: The Eaters of Light) and that the patients in the hospital were in severe pain despite "Razor" trying to convince her otherwise. (TV: World Enough and Time)

She could even notice things the Doctor overlooked, such as the strangeness that a grown man would root through grass and trees for bugs, or that he wasn't old enough for the Doctor's theory to make sense. She was also rather brave, leading her group of flatmates when they were scared. (TV: Knock Knock) She further displayed bravery as she volunteered to combat the oncoming Cyberman from floor 1056 in her cyber form despite the Doctor implying its technology would be more formidable than her own, and stayed to combat the army of multiple fully evolved Cybermen despite knowing full well she would die in the conflict. (TV: The Doctor Falls) She was also brave when confronting the sentient oil, who she thought was an intruder, willingly entering the bathroom to face what was hiding in there. (TV: The Pilot)

She also possessed a strong will power. When the Monks took over Earth, Bill was able to maintain her memory of the Doctor and the world before their rule by using her imaginary image of her mother. (TV: The Lie of the Land) When transformed into a Cyberman, she managed to retain her free will despite being fitted with an emotional inhibitor. (TV: World Enough and Time) The upgrade prolonged her lifespan (though it slowed her down to mere walking speed) and also gave her a cybernetic blaster on the handlebars on her headpiece. Using said blaster by channelling anger, she could blast a gaping hole in a barn and a concrete wall, incinerate another Mondasian Cyberman and could combine with the Master's modified laser screwdriver to disable a fully evolved Cyberman. The upgrade made her resistant to bullets, even when hit at close range. She also retained the ability to cry, a gift her romantically involved classmate Heather granted her to be able to find her. (TV: The Doctor Falls)

Appearance
Bill was a young black woman with an afro, worn in a variety of styles. (TV: The Pilot, etc.) According to the Doctor's sonic sunglasses, Bill was female, was 26 years old by the time of the Monks' invasion, weighed 130.2 pounds, and measured 167.7 cm in height. (TV: The Pyramid at the End of the World)

She didn't like her face because it was "always doing expressions [when she was] trying to be enigmatic". (TV: The Pilot)

When mortally wounded by Jorj, she was partially cyber-converted, given a cybernetic chest-unit. While on the colony ship, she aged, her hair becoming generally messier and less profound. (TV: World Enough and Time) She was also eventually converted into a Cyberman, subsequently looking like one, though she still inwardly perceived herself as looking human. (TV: The Doctor Falls)

Doctor Who The Official Annual 2018
According to the Doctor Who The Official Annual 2018, which is not accepted as a valid source for in-universe articles on this wiki, Bill started sneaking into the Doctor's lectures when she overheard the other students talking about how the Doctor "knew everything about everything" and wanted to know something herself.

Other matters
Bill Potts shares her name with William Hartnell, who was often known as Bill or Billy, while Bill's first love interest shares her name with his wife Heather. She also shares the name with Billie Piper who played Rose Tyler, a companion to the Ninth and Tenth Doctors, as well as the Moment.