The Marquis of Nuts is the son of the Duke of Nuts who appears in the episode "The Duke." He states that he will kill anyone who is trying to hurt his father, and as a result he holds a grudge against Finn. He teams up with Squirrel and they disguise themselves as dignitaries at the Grand Meeting of Ooo Royalty. At the end of the episode, they jump out of their disguises to attack Finn and Jake, the result of which is unclear.
In the episode "Henchman," Marquis of Nuts appears briefly at the party Marceline plays music for. He does not seem to have particular interest in Finn bursting into the castle, indicating he may have moved past his grudge.
Marquis of Nuts has tan-colored skin and brown eyes. He wears a red cape, brown shoes, and a peanut shell as a mask (or armor). It is shown that inside his shell, he is just a single nut on the bottom half of the peanut shell. He carries a lance.
Despite being the son of the Duke and Duchess of Nuts, he himself is technically a legume, not a nut.
The Marquis of Nuts under his peanut shell armor
Marquis of Nuts' disguise at the Grand Meeting of Ooo Royalty