自分の事を主人公だと信じてやまない踏み台が、主人公を踏み台だと勘違いして、優勝してしまうお話です@COMIC

A certain ordinary high school boy always dreamed of being the protagonist in a manga or game. One day, he dies heroically while saving a child and is greeted by a god who promises to reincarnate him as the main character of a novel game world. But—it was all a big, fat lie. Instead of becoming a legendary hero, he’s reincarnated as a weak, disposable stepping-stone character! However, his sheer, unshakable delusion that he is the protagonist drives him to train harder than any actual hero ever would. Before long, his ridiculous efforts start rewriting his fate—surpassing stats meant for mere side characters, rescuing beautiful heroines, and even altering the course of the entire world!