Rebirth: Childhood friend of the heroine

Many people wish to become a character in a novel they like. Side characters are the best ones to reincarnate as. And Ceaser was the same. However, the problem is, the person he reborn as, was supposed to die at that time when Ceaser appears in the fantasy world. The childhood friend of the main heroine, who was supposed to be a stepping stone for the main lead so she can grow stronger. However, now that Ceaser was in that person's body, naturally, he couldn't let himself get killed just after rebirth. But what consequences will he bring by staying alive? And what awaits near the end of this story, about which even Ceaser is unaware?