Tsuzuki's attraction to Hisae could very well be based on a past incestuous relationship with his sister. He makes the statement that Hisae should "dance like she wants/loves him," if they're to dance well together. When asked who taught him how to ballroom dance... he claims it was his sister. (Made more suspect by him later speaking of sharing souls.) Comparing Hisae's design to Luka's... they're very similar.