Gil seems not too much bipolar as very self-loathing. Vincent however is very bipolar.
I think the Glen's soul has been weakened because he lost his five chains. Zai now has the Griffin, and Gil has the Raven. That way he cannot destroy Eliot's soul, just as Jack Vessalius is weaker than Oz in Oz's mind. I wonder if Eliot has a right to use the Jabberwock (the only chain which wasn't distributed to the Duke houses) just like Oz and the B-Rabbit.
It's also possible that parts of Glenn's personality come out in those who are in contracts with his bird-like chains. It's an unlikely reason for Zai's behavior.
But I'm just theorizing madly.
oi, oi - what's up with the bipolar references? i'm bipolar here myself (on medication, mind you!), and i'm perfectly sane! (well, somewhat... XD)
anyways, i'm not mad, i just found it funny that you're saying stuff about being bipolar and i, a bipolar person, happened across it... XD
anyways, i myself think that gil has a very extreme case of bipolar disorder - and no, i don't have any problem saying that. XD have you noticed that he usually has his head/mind all up in a complete mess, and he can't seem to think straight? hmm... maybe a combination of bipolar, split-personality, and multiple-personality disorders?
i wonder what you would call that? XD multi-split bipolar personality disorder? XD that sounds so lame...