Well, yes. I read both of them as being intraverts (I'm not sure about Sephiroth, mostly because I don't think he actually fits into any "human" psychological framework at all... I have no doubt a psychologist or psychiatrist would have no hesitation in labelling Sephiroth as someone with a massive case of antisocial personality disorder) although everyone else in the game is more extraverted. Hence the two of them will tend to isolate themselves and brood to figure out identity issues (and let's face it, they both have those). The other "of course" in the whole business is that everyone else will be attempting to cheer the pair of them up, and try to make them interact with other people more (since that's what works for *them* - ie everyone else - even if it doesn't work for Cloud or Vincent).