Ore no Koigokoro ga Moteasobareteimasu

In order to drive away his persistent previous boyfriend, Yuzuha, one of a set of twins who are popular influencers, declares he's dating his pudgy underclassman Yachiro, who happened to be passing by. Although he was partially caught up in it, the kind Yachiro went along with it and agreed to pretend to be his boyfriend. Yuzuha fully enjoys playing at being lovers with the warm and squishy Yachiro. However, when Yachiro loses weight and drastically changes into a gorgeous man, Yachiro's heart won't stop fluttering... (Source: Kaiousha, translated)