The Extra villain

Kazuma a 17-year-old reader who mistakenly dies and reincarnates into his favorite novel. He didn't reincarnate as the protagonist nor did he reincarnate as the main antagonist, he reincarnated as an extra villain who was meant to be a stepping stone for the protagonist. With the knowledge he has decides to get stronger and survive in this wretched world. Disclaimer: I don't own the book cover!