コイビト・スワップ

"I'm going to take revenge on them." Yuusuke Midorikawa, a high school boy, has a girlfriend named Kayo Yoshioka, who was his childhood friend. After many years, he confessed his long-standing feelings for her and they officially became lovers; but it seems she's becoming estranged from him lately. One day, while he worried over these age-appropriate problems, he witnesses Kayo and another boy who seems to have hit it off with her, leaving a karaoke place by themselves. Yuusuke, unable to put his feelings in order, is met the next day by a pretty classmate named Kotone Shiraki, who introduces herself as the girlfriend of Iketani, the boy he saw with Kayo. The colors of youth will change with the meeting of these two! This is a bittersweet — but more sweet than bitter — youth romantic comedy! (Source: Futabasha, translated)