Yuusha no Ateuma de Shikanai Akuyaku Kizoku ni Tensei Shita Ore: Yuusha de wa Oshi Heroine wo Fukou ni shika Dekinai node, Ore ga Kanojo wo Shiawase ni Suru Tame ni Game Chishiki to Kajou na Doryoku de Scenario wo Bukkowashimasu

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)