Bill Potts is a fictional female Human Companion in the Doctor Who television series who was portrayed on screen by Pearl Mackie. Bill Potts is a dinner lady at St. Luke's University in Bristol, England who sneaks into The Doctor's lectures on Physics. The Doctor notices this is happening and questions her about it. The Doctor manages to persuade Bill to do something with her life and Bill becomes a proper student living with other students.
The Doctor is not the only person who is interested in Bill. An alien life form which can take the form of anyone it pleases is also interested in Bill for other reasons. The other person is a student Heather who has been replaced by morphing alien.
Before the series went live, it was announced that Bill Potts character would be the first openly gay/lesbian character in the series. To many, Captain Jack Harkness was the first openly gay character when he first appeared in the first series when Doctor Who returned in 2015.
Throughout the episodes, we see Bill chasing a love interest. The first love interest of hers is Heather, a fellow student. It turns out that Heather is a polymorphic water creature that Bill had vowed to stay with until the end of time which the creature obeyed. The creature first appeared in the Doctor Who special called Doctor Who - Waters of Mars. Bill managed to break the bonds and the water creature stopped following.
Bill Potts hooks up with Penny and seems to be going ok. Bill invites Penny back to her place, only to be interrupted by the Tardis and the Pope turning up at the most awkward time.
In a number of episodes such as The Eaters of Light and Knock Knock, Bill gains attention from male characters and she has to tell them that she's not interested. I can't remember the other companions having to tell other suitors that they're not interested as they already have boyfriends.
At the beginning of World Enough and Time, Bill is shot in a struggle and taken away by medical personnel. Bill is fitted with an artificial heart. Over the course of the episode, Bill is continuously being upgraded to a Cyberman and is eventually turned into a Cyberman. Bill is able to control her mind and continues to work with the Doctor in order to prevent the Cybermen killing the Doctor. The Doctor is unable to reverse the transformation.
In the follow up episode, The Doctor Falls, although we see Bill, she is actually a Cyberman and she can't accept the fact that she's been turned into a Cyberman. Bill stays loyal to the Doctor and helps in the fight against the oncoming Cybermen.
At the end of the episode, Bill Potts Cyberman has been killed but she looks alive. It seems that she's now become like the Water Creature from her first episode. Heather, the first love of Bill's life returns from a puddle and offers to take Bill off into space which Bill accepts.