Hah, actually I like having someone unpredictable and crazy in a party. Besides, the main goal of this is to have fun anyway, so why not? Although if the others have something against the character, we'll have to do some last minute updates.

Maybe he talked to the others and found out about their booking mishap and is convinced that aliens are manipulating/after them, so he joins them to protect them and uncover the truth that's out there? 

But yeah, I think I'll allow it, but the other players can have a say in balancing. Not that they would, you're on the same side.