Changing the Destinty of the Final Boss

After dying at twenty-five from a terminal illness, a young man awakens within the world of Fantasy Isekai Romance Kingdom (FIRK)—his favorite video game. However, he hasn't reincarnated as the story's conventional hero. Instead, he inhabits the body of his own "Heroic Mode" custom avatar: Kai, a warrior whose power and wealth defy the very limits of the system. His mission is both clear and deeply personal: to prevent Serafina Noire Delacroix—the Grand Duchess destined to become the tragic "Final Boss"—from falling into darkness. In the original plot, the indifference of her fiancé, Prince Leonard, and the betrayal of her friends pushed her into an abyss of loneliness and corruption. Armed with absolute knowledge of the world’s secrets, glitches, and mechanics, Kai decides to intervene at the exact moment Serafina’s life is about to change forever. As he defies the social hierarchies of an empire that sees him as a mere commoner, Kai must contend with the appearance of Saint Karina—who shares an unexpected connection to his past life—and face the conspiracies of demons seeking to claim Serafina as their successor.