Monnus is a descendant of Reshiram himself, where only one of his species is born every generation. He is from nobility, with his father (the previous generation's Reshiram) still the head of the household. To prove himself able to inherit the estate and his family's legacy, Monnus follows his father's instructions and teachings to the letter. He can see through lies and wield strong fire moves, but he cannot lie himself, almost openly sharing his desires. He earned the moniker "slut" from being openly submissive to dominant guys and voicing his desire for wanting them to Bleep him.
He sounds like Naughty, only less friendly :D