Heather (The Pilot)

Heather was a student at St Luke's University in the 2010s. She had a defect in her left iris in the shape of a star. She befriended and developed a mutual crush with canteen assistant Bill Potts. After having become Sentient oil, she gave Bill her tears before leaving. (TV: The Pilot)

Eventually, Heather was able to track Bill to a colony ship, where she had been converted into a Cyberman. Heather was able to revert Bill to a human form, but also turned her into a oil creature like herself. The two decided to see the universe together. (TV: The Doctor Falls)

Life on Earth
Heather hated St Luke's and everywhere else she went, always feeling an indescribable need to leave or go somewhere else. She attended the Twelfth Doctor's lectures on at least one occasion.

Pursuing Bill Potts
As a result of the defect in her eye she quickly noticed that an oil puddle around campus did not properly reflect images like a normal mirror (as a flipped image), as her face was visibly asymmetrical. She showed the puddle to Bill, who later saw her standing over the puddle again. Heather promised not to leave without Bill and was quickly absorbed by the puddle when Bill was not looking. The puddle assumed Heather's appearance and designated her as its "pilot".

The combination of Heather's desire to leave and her promise to not leave without Bill led it to obsessively hunt Bill across time and space in the hopes of taking her with it on its travels. When Bill finally freed Heather from her promise, the oil ceased following her. The image of Heather said a personal goodbye to Bill before she let go, and expressed that she liked her, as well.

Bill later voiced regret at having turned down Heather's offer, and glanced at the TARDIS when the Doctor stated that it was unlikely she would ever see Heather again to suggest that they could find her. Though initially reluctant, the Doctor allowed Bill to join him in his travels, noting that they may one day find Heather again. (TV: The Pilot)

Saving Bill and the Doctor
After the Doctor blew up floor 0507 of the Mondasian colony ship to destroy an army of Cybermen, Heather appeared before a cyber-converted Bill Potts who was standing over the Doctor as he lay dying. She then kissed Bill and turned Bill into the same kind of being as herself Together, they took the Doctor back to his TARDIS, where Heather piloted it. Bill shed a tear on the Doctor's forehead and kissed him. Heather and Bill then departed together, free to explore the universe though Heather revealed she had the ability to return Bill to being human if she ever changed her mind. (TV: The Doctor Falls)

Personality
Heather at first came off as a distant and withdrawn girl who had never been satisfied by her simple life on Earth. This was what caused the sentient oil to select her as its "pilot". She was very observant, easily noticing the puddle wasn't reflecting like a normal mirror thanks to the defect in her eye.

After having been absorbed by the sentient oil, Heather obsessively hunted Bill Potts across the universe as her last thought was her promise not to leave without Bill. The latter later managed to talk Heather out of following her. Heather accepted, albeit tearfully. (TV: The Pilot)

When she finally met Bill again, Heather had regained her old personality and happily kissed her. She also displayed a greater understanding of her abilities and even knew how to pilot the TARDIS. (TV: The Doctor Falls)

Abilities
After merging with a puddle of sentient oil from an alien spaceship, Heather gained the ability to freely travel through time and space. She first followed Bill from St Luke's University to Sydney, then to a distant alien planet, and finally into the middle of a Dalek-Movellan War. She would usually materialize through a reflective surface such as a mirror or a body of water.

Her body also adopted similar properties of the element of water. She could turn into a small puddle of liquid and control its flow in order to travel through pipes or slide under doors. She would then reform into her human appearance, albeit with water trickling from her body. Heather's water form was apparently indestructible. When a Dalek shot her, the blast simply passed through her and her body reformed seconds later. (TV: The Pilot)

Heather could also allow other people to become like her through a similar absorption process. She claimed that she could also restore anyone who had undergone the transformation to their original state and transport them back to their own place and time.

Her transformation also enhanced her intelligence considerably. When she met Bill again, she displayed a far greater understanding of the universe, her abilities, and was able to pilot the TARDIS. (TV: The Doctor Falls)