Motojirō Kajii is a member of the Port Mafia, the criminal organization that controls Yokohama's underworld. He serves directly under the executive Chūya Nakahara. Kajii is known throughout the city as a highly visible bomber, an unusual distinction within the secretive mafia. He is wanted for the bombing of the Maruzen Building, an incident that killed twenty-eight civilians. Physically, he is a tall, slim man in his late twenties with short brown hair cut in even bangs across his forehead. His eyes are concealed behind purple-tinted goggles, and he typically wears a lab coat or similar attire that suggests his scientific inclinations.

Kajii's background is that of a self-styled scientist. He is deeply passionate about physics, chemistry, and the systematic study of the natural world. However, his scientific curiosity is morbidly focused on death, which he describes as a complex music of state changes and the ultimate experiment. He treats his bombing sprees as large-scale physics demonstrations. His personality is extremely eccentric and theatrical. He laughs in a distinctive manner and speaks with great enthusiasm about his theories. He is not secretive about his identity and takes pride in his reputation as a bomber. Despite his seemingly chaotic behavior, he is rationally calculating and views everything through a lens of empirical skepticism. His motivations are driven by a desire to understand death scientifically, to prove theories through observable results, and to challenge those he sees as unscientific, such as religious believers or those who treat life as sacred. He is not driven by malice in a conventional sense but by a detached and absolute commitment to inquiry.

Kajii's role in the story is that of a recurring antagonist and a wild card asset for the Port Mafia. He is deployed for missions that require large-scale destruction or psychological warfare. In his first major appearance, he and Kyouka Izumi hijack a train carrying the Armed Detective Agency members Atsushi Nakajima and Yosano Akiko. Kajii rigs the train with explosives and subjects the passengers to a physics experiment, attempting to induce a fatal state change in them. This confrontation demonstrates his willingness to endanger civilians for his research. He is defeated when Yosano, after being brought to the brink of death, heals herself and turns the tables on him, an experience he finds instructive. Later, during the conflict with the Guild, Kajii is sent to destroy a Guild base. He allows himself to be captured by the Guild member Nathaniel Hawthorne, feigning surrender. When Hawthorne attempts to execute him with his own lemon bombs, Kajii reveals that his ability makes him immune to those bombs, and he uses the opportunity to drop a large quantity of explosives from above, obliterating the base. This act shows his strategic cunning and his value as a combatant.

Key relationships define Kajii's interactions in the narrative. His direct superior is Chūya Nakahara. Kajii shows no particular fear of Chūya and maintains his own eccentric demeanor even in the presence of the executive. He respects Chūya as part of the organizational structure but does not appear to hold any deep personal loyalty. His most notable antagonistic relationship is with Yosano Akiko. Their conflict is rooted in a fundamental philosophical divide: Kajii treats death as a fascinating scientific phenomenon, while Yosano regards death as something to be fought and healed. Their confrontation on the train establishes a bitter enmity between them. Kajii also holds ideological contempt for religious figures, as seen in his mockery of Nathaniel Hawthorne's faith. He argues that science, not belief, is the true path to understanding reality. He has a pragmatic relationship with the Port Mafia boss, Ōgai Mori. Mori regards Kajii's particular brand of madness as a useful tactical resource and gives him the freedom to conduct his experiments as long as they serve the organization's goals. He was previously paired with Kyouka Izumi on a mission, functioning as colleagues.

Kajii shows limited development across the series. He remains consistently defined by his scientific obsession and his manic, confident persona. He does not undergo a significant moral change or emotional arc. Instead, his development is revealed in the deepening understanding of his ability and his strategic role. He is a static character in terms of personality but a dynamic one in terms of tactical deployment, as the narrative finds increasingly inventive ways for him to apply his unique skillset. He represents a pure, amoral form of intellectual curiosity within the criminal world.

Kajii's notable ability is named Lemonade, though it is also referred to as Lemon Bomb. Contrary to initial impressions, his ability does not create bombs. The lemon-shaped explosives he uses are handmade conventional devices of his own design, which are notable for being undetectable by standard explosive sensors. His actual ability grants him complete immunity to damage from any explosion caused by a lemon-shaped bomb. This immunity applies only to bombs of that specific shape, but it makes him effectively invulnerable to his own weaponry. He utilizes this ability in combat by deploying lemon bombs in close quarters and even using himself as a delivery mechanism, walking through his own explosions without harm while his enemies are caught in the blasts. His scientific expertise also extends to chemistry and physics, allowing him to create unusually powerful and stable explosives. He combines this technical skill with his ability to create chaotic and unpredictable battlefield conditions, often referred to as his Lemonade Path technique, in which he scatters bombs around himself as he moves, creating a corridor of explosions that only he can traverse safely.