Kono Naka ni Hitori, Ore no Yome ga Iruこの中に1人、俺の嫁がいる; Kono Naka ni 1-ri, Ore no Yome ga Iru; One of Them is My Wife; Salah Satu Dari Mereka Adalah Istriku

For the sake of finding clues about his younger twin brother who suddenly went missing, the protagonist changed his identity and infiltrated the academy his brother was attending. However, what clearly came to light there was a surprising truth! His brother had multiple fiances...? (Source: Ganma)