Agasa-Kun Wa Yande Iru. summary:

One day, Ichigotani Nanako receives a sudden confession from Agasa Yuki, the genius prince at school who's also her junior in her club. Surprised by this, the natural airhead Nanako blurts out, "I''ll be in trouble if you die though!" and ends up dating Agasa-kun. Anyway, Agasa-kun has an extraordinary obsession and possessiveness, and he has no mercy for any bugs that come near Nanako!