The big news, which was revealed on Saturday at San Diego Comic-Con, marks a monumental shift in representation and role parity for the LGBTQIA community.
Maines will join Supergirl in its fourth season, which premieres this fall, and is set to star as Nia Nal (aka Dreamer). Per Teen Vogue, Nia is described as "a soulful young transgender woman with a fierce drive to protect others." She works at CatCo Worldwide Media, along with Supergirl herself, Kara Danvers.
Speaking to Variety shortly after the historic casting was announced, Maines admitted she was excited and nervous in equal measure.
"I haven't really wrapped my head around it," she said. "It feels fitting to say with great responsibility, comes great power. I'm nervous because I want to do it right."
If Maines seems familiar, you may have seen her in the HBO documentary The Trans List. Or perhaps you read about her — in 2013, she helped set the precedent allowing trans people to use the bathroom of their choice. Maines is also the subject upon whom author Amy Ellis Nutt based her upcoming book Becoming Nicole.
The actor/activist also recently weighed in on the casting controversy surrounding cisgender actress Scarlett Johansson playing the lead role of a trans man in the film Rub and Tug (Johansson ultimately decided to step down from the role).
"Cisgender actors don't take trans roles out of malice," Maines told Variety. "I think it's just failure to realize the context behind having cisgender people play transgender characters because we don't see the same issue with sexuality."
Maines elaborated on the importance of that context, noting that cis actors playing trans characters does the community a major disservice.
"And with trans folks, we have a lot of people accusing us of just playing dress-up for whatever reason, and that's not true. And so having trans people play trans roles shows that we are valid in our identity and we deserve to exist as we do. And so when we have cisgender actors play trans characters, it furthers that stereotype that we are playing dress-up, which is not true," she said.
Right now, though, Maines is celebrating a big win for the community with her Supergirl casting.
"We can be whoever we want, we can do whatever we want, we can be superheroes, because in many ways we are," she told Variety, adding, "We've had trans representation in television for a while, but it hasn't been the right representation."