Rin is actually the character I’m having the most trouble believing right now. I’m pretty antisocial myself, but I can handle myself reasonably well if I’m talking to someone I don’t know well while also with someone I do know is there, because you can bounce off the two as needed (IE: Rin + Riki + any one of the new LB girls). At the same time, she can still go on the “missions” into the girl’s dorm with only mild reservations and interact with complete strangers (I say mild because she always ends up going after putting up a fuss anyway) just because the rest of the original Busters are telling her what to say over the phone. I haven’t entirely finished Rin1 yet, so maybe she gets better, but it’s just a little too odd to me that she still struggles after being around the new girls for extended periods of time, and she is only really entirely alone with the new members once that I can remember. (When she hangs out with the new members solo in the girls dorm before the sleepover.) Not saying her behavior is impossible, but it struck me as a little bit off. Although you can criticize Riki having similar struggles with his own demons.
I’ll admit, this didn’t really bother me until the one scene in the baseball club room where Masato actually did a good thing but still kind of got shit on anyway. I did think the scene was actually really funny up to that point, but then it really put me off and made me feel really bad for him. No wonder he focuses on his muscles: he actually used his head for once and nothing changed.
I think it represents real friends groups as best as it really can. Most friend groups revolve around one or two active personalities driving the more passive ones. Most of the group seems content to interact among themselves until someone brings them together, like in most friend groups I’ve had. I don’t think the chemistry between them is overly embellished, although you could argue that a lot of the super highs and lows of their struggles are dramatized for effect, but so far, all of them are within the bounds of what you might experience in youth, albeit some are more on the extreme side of things.
I may have to catch up with all the new replies as well because, boy there sure are a lot of them. Real proud of the activity this Bookclub is getting 
However, the fact that they can see each other in that sort of relationship without outright rejecting the idea because “Nope I’m not gay” gives more dynamic as to what they really feel for each other. These two are totally dependent on each other, no matter how much or how little they realize it, and that gives so much more meaning to the Little Busters.
