What you have to understand about Wyll is that he's very purposefully Just A Guy. A Dude. He wants his father's approval, live up to his family legacy and get the girl.
Except, he's also been beaten down and left for dead by raiding goblins as a child. This left him with some not so subtle anger issues. To further this flaw, he also spend the rest of his live being just a little too untalented to amount to his own expectations.
Wyll is a bit like a child which suddenly gained the powerful body of a 30 year old. Suddenly, anything is possible. Yet, he's bound to follow a child's Idea of a hero. It's not hard to get under the shell of grandeur and see the unprotected underbelly.
Wyll is an immature character. This contrasts with his supposed chain of past exploits in a way that's a bit too on the nose. I would solve this by making it clear he's been the blade of frontiers for maybe... 2-3 years? Early on, anyway.
However, I also think this emotional vulnerability is why a devil choose him. He's easy to control. Particularly, he doesn't ask the right questions. Wyll doesn't quite know what the right questions are, but thinks he's got it all figured out. He's a knight in shining armour subtly being lead towards his own doom.
I'd prefer it if Wyll isn't recruitable for sure until we've defeated the goblins. My reason for this is simple: the AC. Wyll /must/ come with the medium armour feat and 14 dex. Some of that int could also go towards hp instead. I understand some people would rather have a stronger warlock at expense of roleplay, though. That's propably why it's up to us to fix him.
Nonetheless, him being a forced 5th companion would also mean he gets his scenes with the goblins for sure. It's hard to relate to Wyll If he's never in your party, unlike Gale or Astarion.
