Yes, I am very aware that some of them aren't legally allowed to drink, so you're free to think of it as a timeskip.
Miku es out almost immediately. A sip of wine and she's already begging someone to carry her home.
Rin starts laughing like an idiot and shares embarrassing stories, while people try to shut her up. Every time it happens - she forgets that she's going to regret it later, so she continues and then cries in her pillow the morning after.
Len becomes super aggressive, but like, in a comical way, he'll start jumping on the table, shouting at people and screaming about the good old days and "no Kaito young man, you may NOT take this bottle away from me", despite being younger than everyone else.
Luka is a sophisticated drinker, she doesn't get drunk easily and she doesn't drink much in one go. So therefore she almost never gets drunk, and when she does, it's the same old Luka, but now with a headache.
Kaito gets drunk way too easily and starts crying. When someone tries to help him, he starts crying more because they're being kind to him and he gets overly emotional. He then es out.
Meiko is "fun at parties", she drinks A LOT and becomes very loud and energetic, encouraging others to drink more. She them challenges them in all sorts of games, fights them at the bar, takes them out to karaoke, to amusement park, literally wherever, she could not care less.
Gakupo can handle alcohol very well, even when he drinks a lot, and almost never changes too much, in of personality. The only thing, is that he can become a little more playful and flirty.
Gumi drinks and complains about the taste of the drink every sip she takes, yet still somehow finishes all of the bottle, no one knows why. She then gets up and leaves.

Comments (2)
I could really believe all of these. Especially Len being comically aggressive. I like your imagination.
Wow that's honestly accurate to what I would say too haha XD Except for Gumi, I'd see her as the one laughing at every jokes & running after the animals to inflict them forced cuddles XD