You have reservations about letting this character into the campaign, and the majority of posters in this thread are suggesting you permit it, anyway. I'll be one of the few to go against the grain and suggest you listen to your instinct. You didn't even have to justify it with your lengthy explanation. You know your GM style, the other players at the table, etc... better than the resident "Yes, and..."ers, and your gut is telling you this is a bad idea.