I personally can especially since he has a code. Here's my idea of what kind of secrets he could have.

Okay say that Allison was born with a brother that she can't remember because she was so young when he "died" and the way he died was similar to how her mother died and the story Chris gives is that he took his son who had been bitten out into the woods where he shot and buried him. But the big secret Chris has is that he didn't kill his son.

That or when he was younger he was in love with a werewolf and was told to kill her but didn't and the way it comes out is that a new woman/teacher moves into town and she's suspicious and Chris follows her around and it seems like he's going to kill her but he ends up sleeping with her and having a romance with her.

Honestly I would love to see Allison's reaction to either or both but the latter would be funnier though I think the former would give Allison with her new bitchy persona something to think on though. Especially if her "brother" suffered abuse from many humans and thus had a same outlook on humans as she has towards werewolves.