Choeyakche Mawangui Seuseungi Doeeotda

Story

A growth fantasy where the protagonist, possessed by a character-raising game, turns the weakest Demon King into the strongest! I had been raising a character in the character-raising simulation game "Evil Queen Maker" with the utmost care, only for the final villain to suddenly make her his bride. When I sent a lengthy complaint about the ending… “Then go ahead and create a better ending yourself,” and I ended up possessed by the game’s “Evil Queen Maker”! To escape this game, I have to raise the Demon King and see the ending. But the Demon King I have to raise, Seere,” is only 7 years old.

Synopsis via AniList (English).