Misty Boy(2000)

Suzune never got along with her cousin Yuushin when they were kids. Now that she's in high school, though, he suddenly shows up again after years of separation, and he's like a completely different person -- he's become cool and good-looking! Yuushin says that his grandmother told him, "Protect Suzune," and so now he wants to live with her...!? They're not little kids anymore, and Suzune is running out of reasons to fight Yuushin...