The reason for Finn's behavior? He has psychosis and an "other unspecified persistent mood disorder"
Via likes to pretend she has neither just for the sole purpose of acting like she's better than Finn, when in reality she's either just as bad or 10 times worse. She's masking 100% of the time but her brain is a mess of guilt and stuff that never happened
Bri is too young to have developed either so she's actually relatively normal, in comparison. I think her only real issue is that the seeds for trauma were planted, but she'll never grow up to be able to process it fully. So she's stuck in this limbo state of not understanding why her only friend (Cheetah) had to be taken from her
Finley is not hiding it at all, he's a fucking schizo and proud of it
Whole little gang of finnterparts is just fucked really. And they all blame it on Finn. Except for Finn, who blames it on a 10 year old