18+ Chouko Sennin Haruka

Takamaru is a student at the Ten Treasure Academy, living an ordinary school life, seeking romances which are constantly interfered by Shihoudou Narika (the daughter of his apartment manager). One day, he fell in love at first sight with a transfer student named Takamori Haruka. On that very night, Takamaru and Narika became pursued by a group of mysterious ninja. Just when they thought all hope is lost, Haruka appeared in kunoichi attire. Haruka referred to Takamaru as "young boss", and asked him to do H stuff towards her. This gave her superhuman power that enabled her to defeat the mysterious ninjas.

However, this was just the beginning of a very long battle.

(Source: Alicesoft Wikia)