Boku-H (read the H as ecchi), short for "Dakara, Boku wa H! ga Dekinai" tells the story of pervert extraordinaire Ryosuke Kaga as he gets involved with a grim reaper girl, that needs his perverted energy to survive in the human world, while both search for a special human said to possess great energy.
Ryosuke Kaga is a perverted high school boy attending Momozomo Academy. One day, he meets Lisara Restall standing alone in the rain. Lisara is an elite Grim Reaper, who visited the human world searching for "The Singular Man". Ryosuke makes a contract with Lisara and she sucks Ryosuke's "essence" in order to remain in the human world. This "essence" is his perverted spirit. Ryosuke agrees to help Lisara's search, so as to preserve his own life.