Well as far as personality goes, he's a brat but he's not, like, a complete asshole.
He doesn't lack consideration for other's feelings when made aware of them and he knows to act like an adult when things are serious.
But yeah, as matters stand he's still is a child of nobility, and a puckish boy like sylvester and a girl as frail and interesting as Myne is a recipe for disaster.
He's not a pseudo-noble blue priest, he's the fucking Aub, which is to say the archduke.
Well, I haven't read or watched any other form of this . . . I feel like the new guy is OK. Clearly breezy, probably . . . unwise, but my instincts say basically a good lad.
You know, in all the character and plot stuff, with adoption, murder or kidnapping plots and so on, we're forgetting the important thing--we have movable type, people!