聖騎士の俺が好きになったヒロインが続々とアイツのハーレムメンバーになってしまうんだけど俺の何がいけないのか誰か教えてくれ!!

Synopsis

Ichiro Yamada and fellow student Ryuguin were summoned to another world on their way to the opening ceremony of the school year. With Ryuguin the hero and Yamada the Holy Knight, they decided to conquer the Twelve Labyrinths. All in order to save the world, and to obtain an item that will allow them to return to their original world! Having gained the status of a hero, Ryuguin plays around without making any effort. On the other hand, Yamada, who wanted to return to the original world, worked hard to conquer the labyrinth while being battered and bruised. "The one who supported me was supposed to be the heroine of the same party...!"