Lots of people bringing up Vriska. Does that count as a headcanon? It's so baked into how she acts, to me.

Well, anyway, I don't find myself too strongly assigning trans headcanons to many characters. I think it's more like... As a rational consequence of masculinity and gender being such a present and crushing force in the comic, the cases of it are very cut and dry. It's only reasonable that Equius and Eridan both seem performative and like they need to be set free. It's super normal to get the forcefem mallet out around half the cast.

In seriousness, I might give it to Rose. It's old but gold (thanks Alyson Greaves) and on the whole the Lalondes have A Lot Going On with femininity.