Motojirō Kajii operates as a member of Yokohama's criminal Port Mafia. Publicly recognized as a wanted terrorist, he is responsible for multiple civilian deaths, notably the Maruzen Building Bombing which claimed 28 lives. The path to his Port Mafia affiliation is unclear, though he considers the organization an escape from boredom, offering chances to perform destructive experiments. Details of his background before joining remain undocumented. Kajii exhibits an eccentric and highly excitable personality, frequently expressing himself using cosmic metaphors; he calls the Port Mafia boss the "Grand Marshal of Outer Space." He possesses an intense obsession with science, especially physics, which he uses to rationalize his violence. He views death as a captivating scientific phenomenon and conducts lethal bombings on civilians, framing them as "experiments" to study bodily decay and mortality's inevitability. This outlook causes him to dismiss human life entirely, sometimes pausing confrontations to question victims about their dying sensations. He displays arrogance in combat, mocking opponents for perceived intellectual inferiority, yet reveals cowardice when physically overpowered, as seen during his clash with Akiko Yosano. Strategically, he employs feigned submission to deceive enemies, demonstrated when he pretended surrender against Margaret Mitchell and Nathaniel Hawthorne to set an ambush. His primary ability, Lemonade, allows him to create and detonate lemon-shaped bombs while granting himself complete immunity to their blasts. This power enables him to deploy explosives in confined spaces without self-harm. He supplements this with technical expertise in bomb construction, crafting devices capable of killing individuals or destroying larger structures when clustered. Beyond explosives, he demonstrates knowledge of human physiology, citing conditions like lactic acidosis and neural degradation during fights. His scientific acumen includes strategically placing explosives for ambushes and manipulating environments. Kajii participates in key Port Mafia operations. His first documented mission involves boarding a subway train with Kyouka Izumi to capture Atsushi Nakajima; he detonates bombs killing civilians as a "science experiment" before being defeated by Yosano. Later, during conflict with the Guild, he collaborates with Ryūnosuke Akutagawa to assault a Guild ship. After allowing his capture by Nathaniel Hawthorne and Margaret Mitchell, he exploits their overconfidence to detonate explosives destroying the vessel and incapacitating Mitchell. During Fyodor Dostoevsky's scheme to incite war between the Port Mafia and Armed Detective Agency, Kajii is assigned to defensive operations and successfully repels an initial Agency raid. His appearance features a tall, slim build and short brown hair cut evenly across his forehead. He consistently wears purple-tinted goggles obscuring his eyes. His attire includes a frayed white lab coat adorned with multiple pins—a yellow pin depicting a face, a black pin with a red "X," and an orange pin featuring an exclamation mark—over a white button-down shirt secured by a loose brown belt-like accessory. He pairs this with tattered blue pants, a green scarf, and geta sandals, reinforcing a disheveled, deranged researcher aesthetic. The character draws inspiration from the real-life Japanese writer Motojirō Kajii (1901–1932), who died of tuberculosis. The writer's short story "Lemon" features a protagonist with lung disease imagining the fruit as a bomb in Maruzen bookstore, directly informing the anime character's lemon motif and destructive tendencies. This literary connection contextualizes his preoccupation with mortality and decay.