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.

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, assuming her appearance and designating 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 blowed up floor 508 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 who was near death. She then allowed Bill to become like her and kissed her. Together, they took the Doctor back to his TARDIS and Bill shed a tear on the Doctor's forehead. Heather and Bill then departed together, free to explore the universe. (TV: The Doctor Falls)