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 from Mondas , where she had been converted into a Cyberman . Heather was able to revert Bill to a human form , but also turned her into an oil creature like herself . The two decided to see the universe together . ( TV : " The Doctor Falls " ) Biography . 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 lecture s on at least one occasion . Pursuing Bill Potts . Whilst on campus , Heather discovered a strange oil puddle in a service area behind some buildings . Looking into it , she quickly noticed that the puddle ' s surface did not reflect mirror images like a normal puddle should . She spotted this easily as a result of the defect in her left eye , which made her face visibly asymmetrical . She became fascinated , but also concerned , with the abnormal properties of the dark liquid ; when Bill found her distraught and asked what troubled her , Heather was glad of the chance to show someone else . The two investigated the puddle together . Although Heather was attracted to Bill , her constant desire to leave prevailed and she walked off , much to Bill ' s disappointment . When the two met again the following term , Heather promised not to leave without Bill , but was unexpectedly absorbed by the puddle before Bill could join her to investigate it again . The liquid merged itself with Heather and designated her as its " pilot " . After her transformation , the combination of Heather ' s desire to leave and her promise not to go without Bill led her to obsessively hunt Bill across time and space . She wished to take Bill with her as a passenger on her travels . When Bill finally freed Heather from her promise , the oil - creature ceased following her . Heather said a personal goodbye to Bill before she let go , and expressed her liking for her . She cried at their parting and left her tears with Bill , so that she would know when Bill was in need . Bill later voiced regret at having turned down Heather ' s offer and glanced at the TARDIS when the Doctor stated that it was unlikely they would ever see Heather again , in the hope 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 " ) Heather checks in on Bill at the Übermarket . ( COMIC : " The Great Shopping Bill " ) Saving Bill and the Doctor . After the Doctor ignited floor 0507 of the Mondas ian colony ship to destroy the army of Cybermen fighting for control of the ship , 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 " ) After the Testimony brought Bill back in the form of a glass avatar made with memories recorded before Bill ' s death , Bill stated that the last thing she remembered was Heather appearing to save her . However , she could not recall exactly what Heather did to help her nor did she remember how she returned , causing the Twelfth Doctor to conclude that Bill was not real . ( TV : " Twice Upon a Time " ) 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 . She intensely disliked the defect in her eye and was sensitive to people talking about it . When Bill suggested that it looked nice , Heather irritably responded : " I ' m getting it fixed . " After having been absorbed by the sentient oil , Heather temporarily lost her human personality , and could not communicate , except to mimic the words of others , in much the same way as the puddle had mimicked what it saw . She obsessively hunted Bill Potts across the universe , as her last human thought was her promise not to leave without Bill . The latter eventually 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 to 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 . Her shapeshifting abilities extended to mimicking the appearance of another object or person , such as when she briefly took on the form of the Dalek attacking her before resuming her human shape . ( 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 and her abilities and was able to fully restore her human form , appearing completely dry again . She was able to pilot the TARDIS , explaining to Bill , " I ' m the Pilot , I can fly anything . " ( TV : " The Doctor Falls " )