Bill Potts (Pearl Mackie) is openly gay.
"Yeah, Bill's gay," Mackie told the BBC. "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," she said. "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 recognize 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," she added.
Doctor Who has had gay characters before, like Captain Jack and River Song, but Bill Potts is the first of the Doctor's permanent traveling companions to be gay.
Mackie takes over from Jenna Coleman as the Doctor's female companion. Series 10 is also adding Matt Lucas' Nardole as a full-time companion. Capaldi is departing after this season, though his replacement has not yet been determined. It's also showrunner Steven Moffat's final season.
Doctor Who premieres Saturday, April 15 at 9/8c on BBC America.