[The moment he catches sight of Susato's reaction, Yujin becomes overwhelmed with guilt. This is yet another thing he's kept from her, and it was purely by accident. He's just destined to end up in this situation, isn't he?]

... I'm sorry. I'd rather assumed that he would have told you all about that. But then, I can't imagine it's easy to talk about, either.

[Having demons like that, accompanied with crimes... Perhaps Kazuma didn't want to disappoint Susato, or have her think less of him.]