目が覚めたら、妹の彼氏とつきあうことになっていた件 (Associated Names)

At first glance, he may seem like a player, but at heart, he’s a serious high school boy named Natsuki Hoshii. One day, he swaps places with another Natsuki from a “parallel world” that closely resembles the modern one. In this world, the other Natsuki appears to be both a flirt and in a relationship…