Lady Cassandra O'Brien.Δ17 (pronounced "dot delta seventeen"), or simply Lady Cassandra, was a former human in the far future. Along with other rich and powerful people of the universe, Cassandra was on Platform One, a space station orbiting Earth in the year 5,000,000,000, set to witness the destruction of the planet by the expansion of the Sun. (TV: The End of the World)
According to Cassandra, her father, a Texan, and her mother, who came from the Arctic Desert, were the last to be buried in Earth's soil. She was born on Earth and lived on the edge of the "Los Angeles Crevasse". (TV: The End of the World)
At a drinks party for the Ambassador of Thrace, her future self, using a psychograft to inhabit the dying body of Chip, told her she was beautiful before dying at her side. This memory would stay with Cassandra the rest of her life, as this was the last time someone ever admired her beauty. (TV: New Earth)
She was married at least five times.
Her life was extended through a series of seven hundred and eight plastic surgery operations until she was nothing but a piece of skin stretched onto a frame with eyes and a mouth, connected to a brain in a jar below. The skin had to be constantly moisturised to keep it from drying out. She was guarded by attendants, who sprayed her regularly with a patented moisturising formula kept in canisters. They also served as soldiers when they filled the canisters with acid and killed those who tried to harm her. (TV: The End of the World) She also "force-grew" the clone Chip, who became her devoted assistant. (TV: New Earth)
On Platform One
Cassandra was a guest on Platform One to witness the destruction of Earth by its expanding Sun in the year 5,000,000,000. She presented gifts to the alien guests, including an ostrich egg and a classical jukebox she called an "Ipod".
Cassandra used metal spider robots to infiltrate Platform One's computer systems and the android Adherents of the Repeated Meme to direct attention away from herself as the spiders sabotaged the platform's sun filter. Her original intent was to create the appearance of a hostage situation and collect compensation (with it not being apparent she was the hostage taker) to fund further surgical procedures.
She also ran afoul of Rose Tyler, who was disgusted by Cassandra's narcissistic claims of beauty and purity as "The Last Human" and called her a "bitchy trampoline". In retaliation, Cassandra had Rose knocked unconscious by the Adherents and locked inside a viewing gallery with a sun filter programmed to descend, leaving her to be incinerated. Rose was only saved in time by the Ninth Doctor, who then deactivated the Adherents and uncovered Cassandra as the saboteur.
With her plan exposed, Cassandra considered having her surgeons spray the Doctor with acid. Instead, she ordered her spiders to deactivate Platform One's shields then teleported off the station, leaving it to be consumed, eliminating both the evidence and allowing the shares she held in the guests' rival companies to triple.
However, the Doctor reset Platform One's systems, saving the station, and also reversed the teleportation feed concealed inside the ostrich egg, bringing Cassandra back. In the heat, and without her assistants to moisturise her, Cassandra's skin dried out, stretched and exploded, apparently killing her, although her brain was not destroyed. (TV: The End of the World)
On New Earth
Twenty-three and a quarter years later, Cassandra was living on New Earth. Her brain had survived destruction on Platform One, her eyes had been "salvaged from the bin" by her servant Chip and she was "repaired" with extra skin taken from the back of her previous body. She hid in the basement of the hospital run by the Sisters of Plenitude, tended to by Chip and watching a projected film of her younger self at the Ambassador of Thrace's drinks party to relive her past glories.
When one of her metal spiders discovered Rose and the Tenth Doctor were on New Earth, Cassandra lured Rose to her hiding spot in the basement of the hospital and used a psychograft to transfer her consciousness into Rose, taking control of her body but allowing her old skin and brain to perish in the process. This was in part to gain revenge on Rose and also to further her lifespan, intending to use Rose's body to live on for centuries. She was at first dispirited upon examining the result in the mirror but consoled herself with the more sexual attributes of Rose's stolen form.
Learning of the Doctor's presence inside the hospital from Rose's surface memories and believing he could help her uncover the secrets behind the Sisterhood's miracle cures, Cassandra opted to seduce him under the pretence of being Rose. She made a point of sexualising Rose's appearance in the mirror and concealed a noxious perfume in her cleavage before meeting up with the Doctor in Ward 26, where she exercised her new form by giving him a passionate kiss. Her impersonation of Rose was marred by her Sloane accent, a poor imitation of rhyming slang, and anachronistic technical knowledge. In the end, however, it was her lack of compassion that gave her away to the Doctor when they accessed the hospital's Intensive Care unit and discovered human clones were being used as lab rats for the Sisters' research and medical cures. Her charade exposed, Cassandra knocked the Doctor unconscious with the concoction hidden down her front and sealed him in one of the containment pods.
Cassandra's first instinct upon learning of the Sisters' "plague farm" was to blackmail them; when this failed, she released the clones, who in turn, released more of them and unleashed a pandemic throughout the hospital. Cassandra and the Doctor fled to the lower levels, where she abandoned Chip to die at the hands of the infected and took refuge in her basement hideaway. There, she left Rose's body and possessed the Doctor instead, then climbed up a service ladder to escape the diseased patients.
Unable to open a pair of lift doors at the top of the ladder with the Doctor's sonic screwdriver due to him shielding his thoughts, Cassandra jumped back into Rose's body and was then forced to possess one of the clones when the Doctor refused to help her. When she returned to Rose's body, she appeared to display remorse and sympathy at the fact the clones had never been touched, feeling their pain for herself. Returning to Ward 26, Cassandra helped the Doctor gather a cocktail of intravenous solutions and together they cured the clones using the lift's disinfectant.
In the aftermath of the averted crisis, while the Doctor was trying to persuade Cassandra to leave Rose's body, Chip reappeared. Seeing him as a volunteer (which was confirmed by Chip himself) Cassandra transferred her consciousness to her servant but the stress of the would-be disaster and his near brush with death earlier, compounded with his relative weakness began to tax Chip. As Cassandra informed the Doctor and the now-restored Rose, "He's dying." Though the Doctor offered to help Cassandra survive, she decided that it was finally time for her to die rather than continuing to extend her life further.
As a final request, the Doctor and Rose took her to the party where she had last been told she was beautiful. In Chip's body, Cassandra told her past self she was beautiful, before dying in her past self's arms. (TV: New Earth)
Cassandra was very elitist and looked down her nose at the working-class Rose Tyler, who she considered a "chav". (TV: New Earth) She showed extreme arrogance at times, claiming to be the last "pure human" and considering the new evolved humans "mongrels" due to their intermingling with the natives of the planets they colonised. On Platform One, she was also shown to be selfish, thick-skinned, and devious - willing to sacrifice an entire station of people just for profit. (TV: The End of the World) Her apathetic nature and dislike of "sub-humans" is what ultimately gave her away when she was inhabiting Rose's body, since the Doctor realised that while Rose would care about the new humans' pain, she did not.
Despite her generally callous attitude, Cassandra was occasionally compassionate. After briefly possessing one of the infected new humans, Cassandra displayed genuine distress over their never experiencing touch and all of the pain they were going through. Albeit reluctantly and mainly due to circumstance, Cassandra helped the Doctor save all of the infected people in the hospital and could be seen smiling in happiness until the Doctor told Cassandra that she had helped make the new humans and could not deny them. (TV: New Earth)
Cassandra's fatal flaw was her extreme reluctance to accept death. On Platform One, she rebuffed the Doctor's statement that "Everything has its time and everything dies.", claiming she was too "young". This led her to become callous and highly unapproachable - she watched other people wither and die around her while she grappled onto life with both hands. (TV: The End of the World) The Tenth Doctor was appalled by her willingness to possess other people's bodies just to prolong her lifespan, since it robbed the host of their lives. Only after she realised that Chip's body was dying did she finally accept it was time to die. (TV: New Earth)
As a younger woman, while she still had a full body, Cassandra was far more compassionate, trying desperately to help the strange man who had collapsed after telling her how beautiful she was. This indicated that Cassandra's long life and desperate struggle for survival changed her greatly from the person she once was by making her more indifferent to life in general and to other people in particular. She objectified life and therefore lost sight of the value of it until she was about to lose her own. (TV: New Earth)
In her youth, Cassandra was originally an attractive human woman with "pretty blue eyes" and curled blonde hair who was renowned and admired for her beauty. At the drinks party for the Ambassador of Thrace, she wore a glamorous silver dress. (TV: New Earth)
In her desperate pursuit of remaining the thinnest, most beautiful form possible, Cassandra eventually reduced her body through years of cosmetic and surgical enhancement to nothing more than a flat, translucent piece of skin stretched across a sleek metal frame that was as tall and wide as a man. Her eyes and mouth, both heavy with makeup, were all that remained of Cassandra's face, and her brain resided in a nutrient tank of bubbling liquid at the base of her mobile frame. The skin, which was taken from the front of her body, required constant moisturising to prevent it from drying out and any wrinkles were cut out with a scalpel. By the time she arrived on Platform One, Cassandra had undergone over 708 operations, including a complete removal of her chin, and prided herself on not looking "a day over two thousand". Rose characterised her as "lipstick and skin". (TV: The End of the World)
When Cassandra's original skin was destroyed in the heat of Platform One, she was able to rebuild herself with a skin taken from her posterior. By the time she and Rose met again on New Earth, Cassandra looked much the same as ever, though her frame had grown rusty over time and she was connected to some form of oxygen tank. When she implanted her consciousness inside Rose with the aid of a psychograft, Cassandra's old skin evaporated and she took the form of a glowing streak of pink light when jumping between bodies.
After possessing Rose's body, Cassandra appropriated her enemy's image and youthful physique. She gave her stolen body a mixed review at first, marvelling at having arms and fingers again but dismayed at becoming a "chav". Eventually, however, she came to admire Rose's sex-appeal, noting her "curves" and "nice rear bumper", and compared the experience to "living inside a bouncy castle". When Chip praised her as "beautiful", Cassandra agreed, though once again considered enhancing her new looks with surgery as she believed Rose "could do with a little work".
Having not had a physical human form of her own in centuries, Cassandra adopted a more provocative appearance while impersonating Rose that was at odds with Rose's true nature. She unzipped and later removed the blue jacket Rose wore on New Earth to show off her front, wearing only her purple blouse underneath with the top buttons undone to expose her cleavage. For a brief period, she kept a spray bottle of tranquilizing perfume concealed inside Rose's bra. She also took quite a shine to Rose's blonde hair, at first massaging it into a more volumed style to resemble her own and constantly touching and fondling it with her fingers.
Inside the Doctor's body, Cassandra rejoiced at being male, noting the Time Lord's slim, "foxy" frame, twin hearts and hardly-used "parts". When she briefly possessed one of the hospital's female test-subjects, she was aghast at her diseased, "disgusting" appearance. Finally, inhabiting the half-life clone body of her servant Chip, she described herself as "a walking doodle" and was pleased with Chip's blue hat. When Cassandra died in the arms of her past self, Chip's body was wrapped in a black hooded robe so that his pale, gaunt features and strange tattoos appeared less conspicuous to the party guests. (TV: New Earth)
Behind the scenes
There exists some debate on whether Cassandra is intended to be a transgender woman, based on her line about having grown up as "a little boy". If genuine, it would indeed suggest that she had been raised male. The reference book Doctor Who: Monsters and Villains, which is not a valid source for in-universe articles, lent some credence to this interpretation, as it states Cassandra was born as "Brian Edward Cobbs", in the ruins of the Walsall Apology, in the old Calendar Year of 4.99/4763/A/15.
Conversely, however, it is easy to view the comment as simply another instance of the running gag with TV: The End of the World of the far-future characters using inaccurate terminology to talk about 21st century human life, such as referring to a jukebox as an "iPod". Cassandra's puzzled yet positive reaction to realising that "she's a man!" in New Earth when she possesses the Tenth Doctor also fails to make much sense if Cassandra had indeed been born in a biologically male body.