Sooooooo...it's...not great? So the game in its internal logic views Rin as a boy, not a trans woman. With that in mind, a lot of the conversations surrounding Vincent's decision to fall for Rin are quite frustrating, with his friends being bad about the whole thing. Katherine makes a remark about how if he really fell for someone who wasn't a woman, she herself never had a chance. Where it gets more iffy is that the game basically justifies Vincent pursuing her as "he's willing to overlook something insignificant like gender," which the game then immediately pivots away from to "well, if you ignored gender, I guess you should be able to ignore species." (Rin is an alien) Which.........YIKES! That's my read on it but they basically drop the whole gender thing once they reveal that the REAL thing with her is that she's not human.
As far as how they fall for each other, it's extremely sudden and out of nowhere. Rin just says "I love you" after like 5 days of knowing each other, and Vincent doesn't even believe her at first. On Day 6 if you pursue the Rin route he just pivots as a character to only really caring about her. It's jarring, but also the game is very short and takes place over 8-10 nights (depending on route,) so like...it was never going to be well fleshed out I guess.
