Rat, alias Jiro Nezu, emerges as a core antagonist within the Gentlemen Thief Empire, loyally serving Lady Arsene while operating undercover at Holmes Detective Academy. Posing as a student, he navigates daily interactions with the protagonists, masking his phantom thief identity. Collaborating with accomplices like Stone River and Twenty, Rat employs his pyrokinesis-fueled Toy to wield fire both offensively—detonating explosives—and defensively during heists or skirmishes.

Hot-blooded and confrontational, he frequently clashes with teammates such as Nero, their combustible personalities sparking friction. Though outwardly aloof toward the Milky Holmes team during missions, fleeting glimpses of unspoken concern surface, particularly toward Nero. A running gag sees the protagonists comically misrecalling his alias as "Rabbit" or "Rising Thunder" instead of Rat.

His schemes escalate in boldness, including a plot where the Thieves’ Empire demolishes the protagonists’ garden to incite confrontation. This provocation inadvertently triggers fierce retaliation, underscoring the perils of underestimating their resolve. Balancing dual roles as a phantom thief and undercover student, Rat’s true allegiance remains veiled for much of the narrative, threading tension between covert operations and fleeting camaraderie within the academy’s halls.