I think the reason she comes across as annoying compared to the others is that she's a very... i wanna say immature person, but more like childish - and not in a "grow up already" type of way.
She strikes me as having a fairly severe form of autism, or a similar disorder. The kind where your brain just doesn't work the way most people do, with reduced emotional development and inability to fully understand social cues. In Chii's specific case it is very heavily affecting her ability to maintain and form relationships which go beyond those we see in young children - she's aware that things like romantic and sexual desire exist, but isn't fully capable of understanding them.
The lynchpin of this interpretation is how she is responding to the people around her who HAVE developed enough to engage in the more complex relationships of the "adult" world. She's attempting to isolate herself, because she already feels alienated by her inability to grow, and wants to think it's her own choice to be left behind.
The other characters, when confronted with their lives becoming more complicated, have been forced to grow. But Chii... can't. At least, not as easily as everyone else.
Note that ASD isn't the only reason someone might experience that kind of delayed development, but it is one of the most common, and as far as we know Chii hasn't had any traumatic experiences which would act as a trigger.
I know the answer probably gonna be something like "oh she's just really immature or w/e," but she doesn't read as autistic to me, she reads as asexual. Might be me projecting 'cause I rarely see
any ace rep anywhere (and tbh Chita has been nothing but annoying, but something's better that nothing I guess?).
I'm not a woman so I can't speak on this exact scenario, but I get the perception of delayed development and general not understanding why everyone is pairing off. I spent a long time ruminating on why I didn't really feel the urge for romance/sex (I do know the ace folks can be in relationships, I'm just also aromantic) that seemed to drive everyone else and that made me pissed at myself more than anything as I aged because I knew something was different but I didn't have any grasp on what it could be. That confusion gets worse when you see everyone else seemingly able to grasp the thing you are missing instinctively.
If she
is ace, which again I don't think she is, she's probably just doing some tsundere thing, having your childhood friend - someone you thought was "safe" in some way - approach you with the feelings you don't understand at a fundamental level would probably make me self-isolate and feel "left behind" too. You see your friends experience something you don't
get and seemingly drift away (which is a childish interpretation, but Chita is literally a teenager, a group not known for emotional maturity), but luckily you still have your old buddy...
aaaand looks like he too has what you seemingly lack and is pushing for you to do the thing you don't know how to do that everyone else get naturally. Guess you don't really have a place away from all that unknowable stuff after all. Would probably be easier to just avoid all of it.
That kind of thinking isn't healthy, but if you're dealing with a lot of stress, I think you can understand the self-isolation and lashing out.
