I haven't gotten all of the details, but I think she was involved in some undercover jobs for a while, in support. She's not a fighter, but she's been on the front line far more than she wanted to be. (This is possibly why she owns handcuffs and things like that.)

Gai doesn't know about this, of course. She might drop some hints, but I don't think he can imagine some of the stuff she went through.