It's Nothing

Alex sighed heavily and his breath tickled Claire's neck. "Yeah, I do. Thanks, Claire. I needed to hear that."

So he had been thinking about his past again. It truly was a horrible tragedy that such a gentle soul was raised to be a human weapon. All of his training went against who he was at his core. She had seen the conflict more than once. 

He was such a kind, curious person by nature. He liked knowing how things worked, trying new things, and having fun. Someone like that never should have been a killer but she knew his body count was high even if he refused to give specifics. 

Claire was fairly certain the reason he never talked about it with her was because he was ashamed. He didn't want her to think badly of him because he knew better than anyone how terrified of him she used to be. It was why he swore that ridiculous anime oath in the first place.