Motojirō Kajii/Synopsis

Armed Detective Agency vs. Port Mafia Arc
Motojirō hijacks the train Yosano and Atsushi board. Using the P.A. system, he sarcastically apologizes for the "inconvenience" of a "physics experiment". His "experiment" involves how the passengers will "react" to a sudden explosion. He sets off explosives on the train and assumes the explosion of about two or three people. He cheerfully remarks the next experiment has powerful explosives in the first and last cars, enough to kill all the passengers.

He then addresses Atsushi directly, referring to him as the "main" test subject. He demands Atsushi surrender himself, or else all the passengers die. Yosano proceeds to confront him, stepping into one of Motojirō's lemon bomb traps. After the explosion seemingly kills her, Motojirō mockingly greets her before saying "goodbye". He remarks a slight surprise when she recovers, commenting on how tough modern women are. Yosano remarks it's more surprising such a "celebrity" of the Port Mafia, recalling Motojirō's reputation. At the mention of the Maruzen Building Bombing, Motojirō goes into an ecstatic tangent of how "wonderful" the "experiment" was. He happily views death as something to admire and study.

To Motojirō, "death" and "God" are the apex of science, and represent two concepts science has yet to overcome. His view of death as an "experiment" immediately puts him at odds with Yosano. He challenges Yosano, asking what "color" her death is.

The first round of their battles ends with Motojirō having the upper hand, his onslaught of bombs overpowering Yosano. He gloats over her body, which is covered in broken glass and injuries. He stomps his foot on her back, mocking the Agency for not living up to their reputation. He pulls out a switchblade, informing Yosano she'll die soon. Before she does, he asks her an unsettling question: "What is 'dying', exactly?"

He refers to his obsession with death as an academic interest, calling himself a "scholar". He lists methods of death, stating they're all reversible in a lab environment, but questions why death is, ultimately, completely unavoidable, asking why everyone has to eventually die. Yosano's mocks his inquiry, to which Motojirō asks what some girl from the Agency would know that he, a "lifelong" physics student, does not. Yosano affirms this, saying the answer is simple: he's an idiot.

Enraged, Motojirō stabs the switchblade through Yosano's hand, "thanking" her for the feedback. He drops several lemon-shaped bombs from his coat sleeves. He notes how people claim the brain can stay conscious up to eight hours after fatal blood loss. With a smile, he tells Yosano he'll ask her corpse how it feels to die. He abandons her in the conductor's car, telling her to "enjoy herself".

After the explosions, Motojirō returns to the car, noting the fire spread further than anticipated. The sight of Yosano's body slumped over the window excites him, and he goes to inspect his victory. Consequently, he falls for Yosano's ruse, punched hard enough in the face he flies back into the other side of the car. Shocked, he stutters how Yosano could survive. Yosano doesn't answer, instead of asking which side of his face she just punched. Trembling, Motojirō points to the left side of his face. Yosano promptly punches the right side. He expresses his disbelief once more, listening to her admonish his view of "death" as the apex of science, condemning his callous view of life.

Motojirō remembers stories of Yosano's reputation, the Agency's physician with a rare healing ability. Right after explaining Thou Shalt Not Die, Yosano uses the ability on him, much to his fear. Yosano uses her ability several times to pry information out of him, leaving him dazed and shocked.

The Guild Arc
Motojirō returns, having recovered from his defeat against Yosano, implied to have assisted in killing an assassin sent by the Guild alongside Black Lizard and Chūya. When the Port Mafia's boss makes his entrance, Motojirō promptly bows and pays respect to him, a rare, grim look on his face.

Assigned to eliminate the S.S. Zelda, Motojirō is "caught" by the Guild's personnel, seemingly attempting to flee after being discovered on the ship's wharf, and brought to Margaret and Hawthorne. They confiscated a bag of his lemon bombs, and during interrogation, Motojirō pretends not to know who he is. However, after Hawthorne states he recognizes him from reports, Motojirō retorts that the experiment on the train provided him with valuable data.

Margaret volunteers to kill him, but Hawthorne recognizes his plans of rigging SS Zelda with bombs. Motojirō is surprised to see Hawthorne draw blood and control it and appears to panic when The Scarlet Letter activates. He makes up a ridiculous story of being the "ambassador of the grandmaster of outer space", leading to Mitchell's increased irritation. The Scarlet Letter slashes into his left side, leaving him in pain. Hawthorne mocks his ability, assuming it to be creating bombs. He detonates one of the lemon bombs, throwing it at Motojirō right as it explodes

Just as Guild personnel relocate cargo via helicopter, Motojirō, who survived the explosion unscathed, questioning how Hawthorne hears the voice of God. He goes on listing irrelevant mathematics, claiming science is the only thing that allows humanity to understand God's universe. He points out that, as a clergyman, Hawthorne's faith is vital, however as a scientist, doubt is vital to Motojirō.

Motojirō reveals the truth of his ability, unable to be harmed by the lemon-shaped bombs he creates and proceeds to go into an intense tangent over the lemon. He praises it as the "ultimate in geometry" and "destroyer of this tedious world".

Once more, Motojirō refers to the "outer space ambassador", pointing at the helicopter. Right on cue, a bomb he placed on the cargo detonates, and from the cargo box, dozens of lemon bombs fall. While they fall, Motojirō laughs maniacally, successful in destroying the SS Zelda.

Although they survive the bombings, Mitchell and Hawthorne run into the Guild's secretary, who's not only been killed but had his body propped up by wire, and his face brutally cut open and hollowed out, stuffed with Motojirō's lemon bombs. The Guild members then learn Motojirō rigged much of the territory, so one wrong move could cost them their lives.

The Guild Aftermath Arc
Motojirō encounters Kōyō and tells her that he heard about her attacking a government transport vehicle and notes her strong will by using half a year's worth of funds just to fulfill the task.

Cannibalism Arc
Motojirō participates in the raid against the Agency after Fyodor poisons Mori. He, along with the rest of the Mafia's major Mafiosi, nearly end up in a massive battle against the detectives. However, they are all deceived by Tanizaki's Light Snow.

Soon the raid escalates, with the Agency infiltrating Port Mafia's headquarters. After Mafiosi corner Yosano, Tanizaki, and Kenji with a barrage of bullets, Motojirō takes over, multiple lemon bombs strapped to his chest, and a suitcase in each hand. He erupts into laughter, abruptly speaking of three all-nighters he spent over his research and going as far as bragging about forgetting to put underwear on. His next "experiment", as he calls it, is watching fireworks close up. He opens both suitcases, and a flood of lemon bombs roll out towards the trio. The bombs all explode, in something Motojirō called "the lemon runway", the path in front of him exploding.

His counterattack forces the trio to retreat. In one of his classic eccentrics, he boldly declares he won't allow them to kill the "Admiral of the universe".

After Pushkin's defeat, Motojirō, alongside the Port Mafia's major mafiosi and the Agency, confront him.