My Steady Is a Witch.

Hisashi Seki lives alone in an old apartment building built in a dimensional warp . He's unaware of this until he wakes up to find a gorgeous nude girl, Asami, in his room. They fall in love on sight and decide to stay together in Hisashi's world, however the girl's appearance leads to all sorts of increasingly strange phenomenon. Then an accident shoots Hisashi into another dimension!