Hanaori-san wa Tensei Shite mo Kenka ga Shitai

Ryuusei Narukami spends his days cooped up in his room playing video games, but hides a secret past as an evil dark lord from a fantasy world reincarnated into our world. He has a fateful encounter with the belligerent Hanaori Meteor, the hero from Ryuusei's world who defeated him, now reincarnated as a high school girl.
(Source: Anime News Network, edited)