Doctor Who has introduced several LGBT characters since its return in 2005, such as the pansexual Captain Jack and the married couple Madame Vastra and Jenny, though none of them have ever been part of the main cast.
Speaking to BBC News, Pearl Mackie commented:
"It shouldn't be a big deal in the 21st Century. It's about time isn't it? That representation is important, especially on a mainstream show. It's important to say people are gay, people are black - there are also aliens in the world as well so watch out for them.
I remember watching TV as a young mixed race girl not seeing many people who looked like me, so I think being able to visually recognise yourself on screen is important.
[Being gay] is not the main thing that defines her character - it's something that's part of her and something that she's very happy and very comfortable with."Bill's sexuality will be introduced right at the start of her first episode, The Pilot, in one of her introductory lines of dialogue.
You can see a short interview with Bill discussing this and introducing the companion here: