Well, originally I was planning that she be a baby wyvern that hatched and clung onto the three of us, then maybe one of us would decide to take her as our familiar. She'd be part human, I guess, but like her parents she can freely change between the forms. Really, that second statement is only really necessary depending on who takes her (if they wanted her to be half human), but if you, Umbreon, or I decide that we want to make it a full wyvern, it's fine.
Probably the only thing I ask is who scores the final hit. Unlike the golem, which was a rare monster, this is a boss. The man did say he'd give us all rewards (which is really up to you), the one who scores the final hit will get an incredibly rare Wyvern-based item.
Oh, and one last thing, do you want the child, full wyvern or otherwise?