Captain Nezumi is a Marine officer who holds the rank of captain and commands the 16th Branch of the Marines in the East Blue. His name is the Japanese word for both mouse and rat, a theme that heavily influences his character design and personality. Physically, he is a thin man with a rodent-like face, featuring a thin goatee and several long, wire-like whiskers on his upper lip. He wears a customized Marine uniform that includes a standard cap modified to have flaps resembling mouse ears, which connects to a long, buttoned coat with blue shoulders and a black belt.

In terms of personality, Nezumi is defined by his corruption, cowardice, and arrogance. He is a deeply dishonest and greedy individual who has no integrity, willingly siding with criminals for personal profit. His demeanor is typically smug and entitled, expecting deference simply because of his rank. However, this arrogance vanishes the moment he faces any real physical threat, causing him to quickly grovel and beg for mercy until he can escape to a safe distance. He has a distinctive high-pitched laugh that sounds like a mouse squeaking, often rendered as chi chi chi chi.

Nezumis primary motivation is pure avarice. He has no sense of justice or duty, and he shows open contempt for the civilians he is supposed to protect. This greed leads him into a corrupt arrangement with the Fish-Man pirate Arlong. In exchange for regular bribes, Nezumi agrees to keep the Marines from interfering with Arlongs brutal occupation of the Conomi Islands, effectively allowing the pirate to operate with impunity.

His role in the story is that of a secondary antagonist during the Arlong Park Arc. Acting on a tip from Arlong, Nezumi leads his men to Cocoyasi Village, where he forcibly confiscates the vast sum of money that Nami had been saving for years to buy her village's freedom from Arlong. After the Straw Hat Pirates defeat Arlong and his crew, Nezumi attempts to take credit for the victory and seize all remaining treasure for himself. He and his men are immediately and easily defeated by the pirates, with Nami personally striking him for his role in her suffering. He is forced to return the stolen money and help repair the village.

His most significant action occurs after this defeat. Humiliated and seeking revenge, Nezumi returns to his base and contacts Marine Headquarters. In his report, he falsifies the events, claiming that Monkey D. Luffy and his small crew are dangerous criminals who threatened him. As a result of this report, Nezumi personally requests that a bounty be placed on Luffy's head, which officially makes Luffy a wanted man and sets the course for much of his future notoriety.

Key relationships define his character. His relationship with Arlong is purely transactional, based on mutual corruption rather than loyalty or friendship. He serves as an enemy to Nami, whom he directly victimizes by stealing the fruit of her lifelong labor, and to the Straw Hat Pirates, who humiliate him and for whom he has a deep-seated grudge. Following the Arlong Park Arc, Nezumi does not undergo any significant development; he remains a corrupt and petty officer who fades into the background after fulfilling his narrative purpose of issuing Luffys first bounty. His later fate is not detailed in the main storyline.

Regarding notable abilities, Captain Nezumi possesses virtually none in terms of personal combat. He relies entirely on his rank and the authority it gives him over his subordinates. He is not a fighter and is easily defeated by the main characters, including Nami. His only offensive capability shown is the use of a standard-issue pistol. His true strengths, such as they are, lie in his cunning, his willingness to exploit the system for personal gain, and his talent for bureaucratic manipulation, which he uses to criminalize a pirate who had just saved an entire island from tyranny.