Bill Potts will be revealed as gay, as she will come out in her second line of dialogue in the latest series of the BBC show.
The reveal will be made on 15 April when the show returns.
“It shouldn’t be a big deal in the 21st Century. It’s about time isn’t it?” Pearl Mackie, who plays Bill, told the BBC.
“That representation is important, especially on a mainstream show.”
She continues: “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.”
Mackie’s portrayal of the first out gay companion does not mark the first out LGBT character on the show.
River Song and Captain Jack have both been out as LGBT.
Mackie takes over as the new companion after Jenna Coleman’s exit from the show.
She says it is “kind of insane” to be the centre of attention since she was announced as the new companion last year.
“My Twitter follower count went from 400-and-something to 16,500 in about two hours, so that was pretty mental,” she said.
“But it’s been really nice, everyone seems to be really excited. It’s sort of like being welcomed into a family and all the fans have been really welcoming.”