難波康介イカせてなんぼ!

Kosuke is a graduating high school student who lives alone in Shinjuku. He has a secret passion for his classmate Miki and is anxious to confess his feelings to her before the final exam day. But as soon as he settles a date with Miki she dissapears and an unknown floating girl appears in a flash, then stumbles and looses consciousness. Girl is declares that a "holy war" is approaching and that the fate of humankind and Miki depends on Kosuke now. It turns out that legendary princess Kaguya actually married one of her five guards and while Miki is a descendant of Kaguya-hime, Kosuke comes from the line of her husband. In order to save Miki, Kosuke needs to sublimate passion of other girls.

[From vndbreview]