The Double Revenge

Kazuki — the student council secretary, is dating Akari — the younger sister of the student council president, but lately they've been unable to meet up due to her reasons. The student council president also has an older boyfriend, but when she says their relations have similarly gone cold, she teases him with the words "Why don't you switch to me instead?"; Kazuki got rejected by her in the past.

Seeing a fellow comrade in him who has nothing else to do, she invites Kazuki to go watch a movie with her and her boyfriend. But near the place where they were supposed to meet up, they both saw Akari together with said boyfriend entering a love hotel.

(Source: DMM)