"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 added: "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."
Gay and bisexual characters have featured in Doctor Who before, such as Captain Jack and River Song, but this is the first time the Doctor's permanent companion has been openly gay.
Although Captain Jack - played by John Barrowman - travelled with the Doctor for a number of episodes, he was not a full-time companion in the traditional sense.
Mackie said it had been "kind of insane" to be the centre of attention since she was announced in the role last April, despite not being seen on screen until this Easter.
"My Twitter follower count went from 400-and-something to 16,500 in about two hours, so that was pretty mental," said the actress, who is taking over from Jenna Coleman as the Doctor's companion.
"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."
The actress, who was performing in the West End production of The Curious Incident of the Dog in the Night-Time at the time she was cast, said the role of Bill was "the biggest job I've ever done".
"Looking at previous alumni, it's only done wonderful things for their careers - Matt Smith is phenomenal in The Crown, Jenna Coleman has just gone on to do Victoria and Billie Piper is doing incredible things," she said.
"For me if I even get to do [a small amount] of those kind of things it would be incredible. It's an amazing platform for me and it opens a lot of doors that wouldn't necessarily have been open before - it's very exciting."
This series of Doctor Who will be Peter Capaldi's last, as the actor is due to hand over the key to the Tardis during this year's Christmas special.
Speculation is mounting over who will take over the role, with Fleabag's Phoebe Waller-Bridge among the potential candidates.
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