The trivia section states:
It was confirmed by Morgan Berry, Thirteen's voice actor in the english dub, that Thirteen is indeed androgynous, meaning they use the pronouns they and them. However, their assigned at birth gender is unknown.
How does the claim of the English voice actor confirm the character's pronouns in-series? He is just a voice actor, not the script writer. In the japanese language Thirteen is referred to as a he.