Supergirl is set to become the first major TV series to feature a transgender superhero.
Transgender actor Nicole Maines, 20, will join the US show in its fourth season in the part of Nia Nal.
Speaking at the Comic-Con event in San Diego, Ms Maines said: "A lot of shows are introducing trans characters to their casts because this is such a hot topic issue right now and it's so important in our society.
"You know it seems fitting to have a trans superhero - Supergirl stepped up and I stepped in."
Supergirl, which is based on the DC Comics series, features Superman's cousin Kara Danvers, otherwise known as the 'girl of steel'.
Ms Maines famously fought a legal case in 2014 for her right to use the girls' toilet at her school in Maine.
The US state's supreme judicial court ultimately ruled in her favour and deemed the school had fallen foul of a human rights act that gave transgender students access to the toilet of the gender with which they identified.