Fat Shell? Her Warrior Soul!

The Martial God was unbeaten until the Demon God laid a trap. The trap worked, and the Martial God's spirit passed by accident into the body of Mira Quinn, a fat woman whom everyone around her called useless and laughed at. Mira is forced to enter the kingdom's tournament. Her father is afraid for her, so he rigs the trials behind her back to keep her safe. The cheating is discovered, causes a scandal, and leaves a misunderstanding hanging over her. Then Mira fights for real. She beats two rivals outright, and the ones who laughed go quiet. She is promoted, and then promoted again, up toward the post of general, which no woman in the kingdom has ever held. The Crown Prince sets himself against her. Her father keeps backing her from behind the scenes.