My Revenge as a Rat

Flynn Forrest is murdered by his parents' adopted son, Leo Forrest, and his girlfriend, Sadie Jennings. Reborn as a rat, he acquires an Emotion Points System. He collects Emotion Points to evolve, teasing four Campus Belles. He faces crises, but saves his parents. In the end, he evolves into the Void Beast, kills the Flood Dragon, and completes his revenge.