Captain Nezumi (ネズミ大佐 Nezumi Taisa) is a Marine Officer of the 16th Branch, in the East Blue. His appearance and manner are rodent-like (as “nezumi” is Japanese for both “mouse” and “rat”), and he is arrogant and decidedly corrupt. Nezumi takes bribes from Arlong, so the Fishman pirate can keep a low profile. Acting on a tip from Arlong, he uncovers the treasure that Nami had been hoarding in order to buy back Cocoyashi Village and confiscates it from her. After Arlong is defeated, Nezumi attempts to intervene against the Straw Hat Pirates, but he is quickly defeated and forced to return all that he stole.
His report back to his superiors is what prompts the Marines to assign Luffy with his first bounty.
First Appearance: Chapter 69; Episode 31

(Source: One Piece Wikia)