Extra C is Secretly Overpowered

It was only last year that I realized that the world I had been reincarnated into was the world of the Romantic Comedy book. What's worse, the person I had reincarnated as was someone featured in the book. A running gag character who the Main Characters ran into at the most random times, of no use, and who would disappear as quickly as he appeared—an Extra C. Last year. A year before starting High School. It was already too late. Extra C had already become...