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.
First Morgan Berry is a woman, second she was told by the director that the character is that way and thus to voice the character in that way and third Thirteen is in fact NOT referred to any gender in Japanese. Any scanlation that referes to Thirteen as he or she are incorrect guesses. Even Viz messed that up since they have to make guesses to make sentences flow better (they also referred to Nana as a he for instance in All Might's dialouge with Izuku when he admits he wasn't born with a quirk but his master was so they aren't perfect). HostLifeForm (talk) 20:13, February 17, 2018 (UTC)
That was still just the director's instruction. Not the actual series creator's confirmation. As for the manga pronouns, I am honestly not quite sure myself anymore. I will try to find the official source.
Agreed, the original manga is the go-to source material.