From DMP:

When high schooler Sayaka awoke one morning, she found an unexpected guest at the family kitchen table - Kai, a cool, handsome, aloof, and so-called family friend of Sayaka's father. His temporary stay in her household and attendance to Sayaka's school is more than she's bargained for.

But what secrets does Kai hold? What are his true intentions for his sudden appearance into Sayaka's life? Where does he disappear to every night? And what's up with that black cat?