I'm the Strongest in This Zombie World, But I Can't Beat This Girl!, Vol. 1 — Ryou Iwanami

Part human, part zombie, all horny high schooler!When a sudden zombie outbreak ravages Japan, Yuuma Kousaka is bitten while trying to escape. Afraid of dying, he sits on a riverbank, waiting for the zombie virus to take over, where he runs into Haruka Hyuuga, the younger sister of his old school friend. Haruka takes pity on Yuuma and offers to fulfill his dying wish by becoming his girlfriend, giving him one final, irreplaceable memory to take with him to the grave -- letting Yuuma touch her breasts. But when he finally transforms into a zombie, he retains his consciousness! Now that he can transform between zombie and human, Yuuma might very well be the strongest person alive in this new world…but he's powerless against Haruka, who knows all his weaknesses!