I'm a villain within the hero's party - Chapter 1 Part : Ren Restes

Kaito Nakamura, a shy university nerd, thought he’d found love when the stunning Aiko chose him as her boyfriend. Their sweet, fluffy romance fills his lonely world with hope—until he discovers she’s cheating on him with multiple guys, reveling in the taboo thrill of betraying someone so innocent. Heartbroken and humiliated as explicit photos of her circulate online, Kaito’s tears are interrupted by a mysterious popup: the “Revenge NTR System Beta.” Promising retribution but with a chilling condition — he can’t break up with her.Contains Ntr, read at your own risk

On the day of her escape from prison, she had died unexpectedly and was reborn into the body of Madam Huo.

Three years ago, the animals and plants on Earth underwent crazy and unexplainable changes that included transformations, reversions, and the addition of otherworldly species. And it was the start of a brand-new type of profession in the new generation of humanity – Monster Trainers, people who specialized in taming monsters.

On his 18th birthday, the protagonist of the story, Gao Peng, suddenly received the ability of being able to see the attribute data frame of monsters. With his ability, he aspired to become the strongest monster trainer that ever lived while looking for a method to revive his parents!

- Description from Novelupdates