She was portrayed by Pearl Mackie.
Not much is known about Bill's early life apart from that she was born around 1991 to an unnamed woman, who later died.
Following her mother's death, Bill was cared for by a woman named Moira.
Upon growing up, Bill got a job working as a canteen worker at St. Luke's University in Bristol. When off-duty, Bill (despite not being a student) would watch lectures by the Doctor. He noticed Bill's presence at the lectures and offer to tutor her. During this point, Bill met and developed a crush on a female student named Heather, who promised Bill to never leave her. Afterwards, Heather became the host of living oil, that was left behind on Earth, and pursued Bill due to the promise. After Bill releases the promise, Heather leaves. Upon returning to St. Luke's, the Doctor planned to erase Bill's memories of what had happened, but she convinced him not to.
Afterwards, Bill joined the Doctor on travels around the universe along with Nardole (who acted as butler to the Doctor, having previously worked for River Song). In the episode "The Lie of the Land", aliens known as The Monks took over Earth and made the majority of the population believe that they have been on Earth since the beginning. Only Bill knew the truth and reunited with the Doctor and Nardole to defeat the Monks.
In the episode "World Enough and Time", Bill arrived on a spaceship with the Doctor, Nardole and Missy (the female incarnation of the Doctor's childhood friend turned enemy The Master), where she was shot in the chest, before being taken to the bottom of the ship to be healed in the hospital. However, due to the ship being near a black hole, time at the top of the ship (where the Doctor, Nardole and Missy were) moved slower than at the bottom (resulting in what was minutes at the top of the ship to pass as 10 years at the bottom). During her time in the hospital, Bill befriended one of the staff Mr. Razor. However, one night Razor turned against Bill and allowed her to be converted into a Cyberman. Upon arriving at the bottom of the ship, the Doctor and Nardole encountered the now conversted Bill and also see Missy team up with her previous incarnation (who had been disguised as Mr. Razor).
In the following episode "The Doctor Falls", it is revealed that during the Monks' rule over Earth, Bill learned to resist and thus retained human emotions after being converted (as usually Cybermen lose their emotions upon being converted). She also saw herself as human whilst the Doctor and others saw her as a Cyberman. After the Doctor blew up a Cyberman army, Bill came across his unconscious form and wept. Shortly after, Heather arrived (having tracked Bill via her tears) and freed Bill from being a Cyberman and made her into a oil being like her.
The two then took the still unconscious Doctor back to the Tardis. Heather then gave Bill a choice; Become human again and live a normal life or travel the universe with her. Bill chooses the latter and bids a sad farewell to the Doctor and leaves her tears on him, saying "when there's tears, there's hope", before leaving the Tardis with Heather.
Bill reappears in the 2017 Christmas special "Twice Upon A Time", where it is revealed that at some point, Bill's memories were retained by a practice called the Testimony who placed the memories into a glass avatar, which could change to and from resembling Bill. In the special, Bill reunites with the Doctor (who was refusing to regenerate) meets his first incarnation and a World War I captain named Archibald Hamish Lethbridge-Stewart (a relative of the Doctor's friend Brigadier Alistair Gordon Lethbridge-Stewart). Throughout the special, the Doctor, believing that Bill had died and had not witnessed her transformation into an oil being, could not believe that Bill was really there. At the end of the special, Bill restores the Doctor's memories of his former companion Clara Oswald, causing him to see a vision of her. Bill along with a glass avatar of Nardole bid farewell to the Doctor, before teleporting away and the Doctor returns to his Tardis to regenerate.
- Bill's departure is slightly similar to that of her predecessor Clara Oswald.
- Both have tragic fates (Bill becomes a Cyberman whilst Clara was killed by the Quantum Shade).
- Both were later revived (Bill was freed from being a Cyberman by Heather and became an oil being whilst Clara was extracted from her fixed point of death by the Doctor).
- Both continue to travel the universe with someone else instead of the Doctor (Bill continued to travel the universe with Heather whilst Clara travelled the universe with agirl named Ashildr (whom the Doctor made immortal during the Viking times).