Otasuke Chara ni Tensei Shita no de, Game Chishiki de Musou Suru

The main character is reincarnated as Ash, the character of the game he was playing. However, Ash was a so-called "helper character" who helped the main character in the early stages of the game and was later killed by the enemy. The only thing he can use to live as Ash is the knowledge of the game. In order to change the fate of himself and his favorite character, the villainess Mercy, he sets out to conquer this world.

(Source: Brave Bunko, translated)