Magical Explorer, Vol. 4 (light novel)

After clearing forty layers of the Tsukuyomi Academy Dungeon solo in record time, Kousuke is finally invited to join the Academy's most politically powerful student organization: the Three Committees. As Kousuke continues to do everything he can to guide his friends to a happy ending, he senses that something strange is happening with Iori Hijiri's younger stepsister, Yuika. But what should be an easily resolvable event quickly gets out of hand when Kousuke realizes that the story of the game has branched off into uncharted territory!