Nisekoi Doumei.

Aizawa-kun becomes the narrator’s fake boyfriend after they discover their partners betraying them. Their secret arrangement begins one day after school, when they witness his girlfriend and the narrator’s boyfriend stabbing them both in the back. Pretending to imitate the relationship they have seen, the pair become co-conspirators in a sweet but painful romance. As they keep up the lie, the narrator feels a pang of genuine affection beneath their staged relationship.