Magical Explorer, Vol. 4 (Light Novel): Reborn as a Side Character in a Fantasy Dating Sim (novel)

After clearing forty layers of the Tsukuyomi Academy Dungeon solo in record time, Kousuke is finally invited to join Tsukuyomi's influential Three Committees. As he continues to do everything he can to ensure he and his friends get a happy ending, Kousuke senses that something strange has happened to Iori Hijiri's younger stepsister, Yuika. But what should be an easily resolvable event quickly gets out of hand when Kousuke realizes that he's entered a new route that not even he knows about!