Highschool DXD: Pervert Past, Kingly Present.

Issei Hyoudou was the resident pervert of Kuoh Academy, a guy whose mind never left the gutter. But a mandatory trip to the Familiar Forest changes his destiny forever. When he stumbles upon a mysterious, defenseless girl named Hakuno Kishinami under attack by vicious familiars, Issei ignores his fear and moves to protect her. In that moment of life or death, a dormant, ancient soul awakens within him-the King of Heroes, Gilgamesh. Now bound to Hakuno as an Archer-class Servant, Issei finds himself caught between two completely different lives. On the outside, he is slowly morphing into a proud, regal king who commands absolute respect and wields the boundless treasury of the Gate of Babylon. But on the inside? He's still the same old Issei, fighting a losing battle to keep his shameless thoughts from slipping out. The supernatural world of Kuoh expects a weak human boy, but they are about to face the golden brilliance of a true King.