^ I could actually see that being the reason they get close in the first place. Like, Tojiko finds out who Mokou is, and confronts her about it. After the initial fight, the two of them manage to work past their differences and eventually become friends. And some time after that, they slowly realize they're attracted to each other, but are scared to make the first move, which leads to this picture.
Come to think of it, another reason Tojiko x Mokou would work, even if they were just friends, would be that neither of them would outlive the other, since Mokou is immortal and Tojiko is a ghost.