The character that underwent extensive medical procedures in order to convincingly present as female.
And I'll repeat it again, because apparently I need to. Whether or not Finkle/Einhorn is actually trans is not the point. I'll try to demonstrate it nice and simple.
Version 1: Finkle is faking
- Presents the idea of a man pretending to be a woman to do crimes - a transphobic idea that has been used repeatedly to deny trans woman fair treatment.
- Presents exaggerated vomiting after finding out you accidentally did a gay thing as the "normal" reaction (since it's not just Ace, an outlier, that has that reaction) - this is homophobic. It's literally stating that a man's lips touching another man's lips is "sickening".
- Ace's reaction to discovering the "truth" of Finkle's identity, that entire scene playing out like he had just been a victim of rape... classic gay panic. "The gay man tricked him into kissing! That's comparable to rape!"
Version 2: Einhorn is really trans
- Has characters, including the main character with whom we are supposed to sympathise and agree, insisting that the trans woman isn't really a woman because she still has "Sargeant Winky". There's two ways to take that. It's either saying "you're not a real woman unless you're born a woman" or it's saying "you're not a real woman unless you've had the surgery". both of those are transphobic.
- Presents exaggerated vomiting after finding out you kissed a trans person as a normal reaction. I should think that's pretty obviously transphobic.
- And Ace's reaction to the revelation. Treating the fact that he unknowingly kissed a trans woman as equivalent to having been raped is super gross.
Hell, let's go past all of that and just look at Finkle/Einhorn as a character.
Either he's a gay man who pretends to be a woman to "trick" straight men into sexual encounters, or she's a trans woman who predates on unsuspecting straight male victims.
One of those is homophobic as shit, the other is transphobic as shit.
I'm sorry - I love the movie, but its big twist has aged phenomenally fucking poorly. Something I believe even the writers themselves have admitted.