勇者の当て馬でしかない悪役貴族に転生した俺 ～勇者では推しヒロインを不幸にしかできないので、俺が彼女を幸せにするためにゲーム知識と過剰な努力でシナリオをぶっ壊します～

The protagonist, who repeatedly plays the game in search of a happy ending for his favorite heroine, Selvia, but is angered by her treatment as merely a member of the hero's harem, wakes up one day to find himself reincarnated into the game world. However, he has been reincarnated as Cain, Selvia's fiancé, a villainous nobleman who torments her and is defeated early in the game. If he does nothing, Selvia will eventually be taken by the hero and Cain will be killed, so he decides to break the game's scenario and seize a future where he can be happy with Selvia. (Source: Tokuma Shoten, translated)