I've tried to challenge Kevin's relationship to Peter on as many levels as possible.
He's loosing his friend peter because peter is (possibly) turning into a girl and at the very least an animal person...
His friend has found a closer relationship with another animal girl.. making him less relevant somehow.
And of course Kevin has his own changes to deal with and nobody he feels he can confide in.
So yeah he's probably jealous, scared, envious.. it's a lot to deal with. He's actually become all the things he hated most as a result of this jealousy and has yet to realise that.
