Lol when she first came, to prevent casualties, he promised her that if she behaved, he would not out her as a Titan nor speak of her betrayal to anyone in the mansion. Basically just let her live like everyone else as long as she didn't cause trouble.

Recently he's been emotionally manipulating her to get her stay in line, since he realizes she has a soft spot for him. He's really, really terrible, and I love it.