Marcell is LKJ's cousin and personal chef from Brazil. He also runs the famous resturaunt, Ratatoing. Marcell uses stolen human food in his own food, which is unheard of in the rat food industry. He never gives this away for some reason.
He is responsible for making the secret recipe for Potato Knishes LKJ always follows. However, his master seems him as expendable. He was mauled to death by Scar when he came to live with the rest of the Brotherhood of Chaos at LKJ's Castle.