Rat, alias Jiro Nezu, operates as a primary antagonist within the Gentlemen Thief Empire under Arsène's command. He forms part of the Three Cards, demonstrating fierce loyalty to Arsène, who united him with fellow thieves Twenty and Stone River despite their initial mutual hostility.

His personality radiates high energy and a fiery, hot-blooded temperament. This aggression often drives the group's offensive heist operations but sparks frequent clashes with comrades like Nero, sometimes escalating into mission-compromising conflicts such as the Yokohama Museum infighting. Despite his antagonistic role and initial coldness toward the Milky Holmes detectives—encountered undercover as their Holmes Detective Academy classmate—he occasionally reveals subtle, indirect affection toward them.

Rat wields the supernatural pyrokinesis ability known as Toys, primarily employing it to ignite explosives for offensive assaults and defensive maneuvers during thefts and confrontations. His combat tactics heavily feature bombs and fire-based attacks, which opponents sometimes counter or exploit.

A recurring comedic element involves the Milky Holmes members consistently failing to recall his name, mistakenly calling him "Rabbit," "Rising Thunder," or other variations despite direct interactions in both his civilian and phantom thief roles. His antagonistic actions include participating in the destruction of Milky Holmes' garden—an act provoking their intense retaliation—and collaborating in schemes to publicly humiliate or expel them from the academy. He was also falsely implicated in a staged attack leaving a "dying message" designed to frame Milky Holmes for his supposed murder.