Class no Inkya ni Kareshi no Sainou ga Arisugiru

After getting dumped in the worst way, a heartbroken girl discovers that her quiet, unremarkable classmate is actually insanely good-looking. To make her ex regret his decision, she (half-forcefully) convinces him to pretend to be her boyfriend. But little does she know—his natural boyfriend-like gestures keep making her heart race?

(Source: Pixiv Comic, translated)