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Supergirl is set to become the first ever live-action TV series with a transgender superhero.

It was announced at Comic Con in San Diego, California, that actor and campaigner Nicole Maines will play the character Nia Nal, who is known by the superhero name Dreamer.

Ms Maines is transgender herself, and appeared in the HBO documentary The Trans List.

"It seems only fitting that we have a trans superhero for trans kids to look up to," she told the audience at Comic Con.

In an interview with Variety magazine, she added: "We can be whoever we want, we can do whatever we want, we can be superheroes, because in many ways we are.

"We’ve had trans representation in television for a while but it hasn’t been the right representation."

She added: "Having trans people play trans roles show that we are valid in our identities and we exist."

CW's Supergirl, which begins its fourth season on October 14, described the character Nia Nal as "a soulful young transgender woman with a fierce drive to protect others".

Fans hailed the announcement on Twitter, with one saying that Ms Maines is "already a superhero", citing her trans rights activism.

Maines brought the case Doe v. Clenchy before the Maine Supreme Judicial Court in June 2013, which set a precedent for allowing trans people to use the bathrooms of their choice