Atsushi Nakajima/Synopsis

Background
Not much is known about Atsushi's background thus far. He grew up in an orphanage, where he suffered from severe abuse, both verbal and physical, from his caretakers, as well as social isolation from the other children. This history of mistreatment has evidently taken a huge toll on Atsushi's self-worth as an adult and has caused him to constantly question the value and usefulness of his life.

According to the headmaster, Atsushi's parents dumped him in the trash. This headmaster becomes the foundation of Atsushi's warped self-worth. The abuse (presumably severe especially because of the tiger rampaging at night) led the headmaster to chain Atsushi in a cell with little to no food, and forcibly injecting him with either sedatives or nutrients. The headmaster emphasized Atsushi's hatred towards him, seemingly even encouraging it. Alongside the headmaster and other caretakers' verbal and physical abuse, Atsushi suffered at the hands of the other orphans. Often times, the other orphans threw Atsushi under the bus when they got in trouble, as the orphanage ran on some sort of point system that affected if and when they were fed.

Once he hits 18, Atsushi was kicked out of the orphanage because, though he was unaware of it at the time, he was transforming into a white tiger under the moonlight due to his ability and causing chaos.

Introduction
Aimless and starving, Atsushi arrives in Yokohama two weeks after being kicked out of his orphanage. Although plagued by the orphanage staff's cruel words, he refuses to submit and die. Although reluctant, he determines he'll have to steal in order to survive. He intends on waiting for the next person to pass him by the Tsurumi River's bank. However, the next person he comes across is a young man drowning in the river. Atsushi ends up saving the man.

Quickly coming to, the man is displeased with being saved. Atsushi responds with great confusion, and the man clarifies he was trying to commit suicide by drowning. However, at the same time, he wanted his suicide to go without disturbance. As Atsushi got entangled in his attempt, the man intends on doing something in return. He hears Atsushi's stomach growl, and relates to the feeling, claiming he hasn't eaten in a few days himself. However, the river swept up his wallet, leaving him without money. Soon after, Atsushi meets the man's colleague. Eventually, the two strangers end up taking Atsushi to a restaurant to eat chazuke in exchange for saving the suicidal man's life. The man finally properly introduces himself as Osamu Dazai.

During the meal, Atsushi learns Dazai and Kunikida are detectives of the ability-using Armed Detective Agency. He also learns their current investigation involves a giant white tiger that Atsushi remembers as one that has been following and taunting him for much his time after being kicked out. After forceful persuasion via Kunikida, he reveals the tiger was the cause of severe damage to the orphanage he was kicked out of. Now, it appears the tiger followed him all the way to Yokohama.

Using the reward money as a bribe, Dazai pulls Atsushi into a plan as bait to capture the tiger once and for all. However, during their wait, an agitated Atsushi is at a loss for words when met with Dazai's complete nonchalance towards the tiger's looming threat. As moonlight pours into the abandoned building, Atsushi himself transforms into an aggressive white tiger. However, Dazai uses his ability to nullify the tiger, returning Atsushi to normal, albeit unconscious.

He wakes up in a dormitory and is called by Dazai, the one who set up his lodgings. Dazai claims he is about to die, and Atsushi quickly moves to check the scene, only to be met by yet another failed suicide attempt of Dazai's. After helping Dazai, Atsushi meets back up with Kunikida as well, who informs the duo of a mad bomber holding up the Agency.

At the Agency's office, the bomber turns out to be a young, angry man holding one of the part-time clerks hostage. He demands to see the Agency's president, otherwise, he'll detonate the bomb. Dazai and Kunikida end up playing rock paper scissors to determine who tries to diffuse the situation. After losing, Kunikida approaches the man; however, the man recognizes Kunikida and his ability, demanding he does as he says or else the bomb detonates. At a stalemate, Dazai makes Atsushi approach the young man to take care of the situation himself.

Terrified and nervous, Atsushi ends up going on a personal rant. He understands both he and the young man have nowhere to turn to, and urges him to try finding work or another purpose, instead of harming others because of his own misfortune. His intensity disarms the young man's defenses and Kunikida subdues him with Lone Poet. Relieved, Atsushi is caught off guard by an unseen figure pushing him, making him fall on the detonation device. Panicking, Atsushi uses his own body to lessen the bomb's impact, much to Dazai's shock.

The bomb, however, does not go off. The bomber stands next to Kunikida and Dazai who both comment on Atsushi's impulsiveness. The clerk, Naomi Tanizaki, hugs the bomber - her older brother, completely fine and amicable with him. Confused, Atsushi questions the situation, to which Dazai reveals it to be an entrance exam for Atsushi's position in the Armed Detective Agency.

Thus enters the Agency's director, Yukichi Fukuzawa. He confirms Atsusi passing the entrance exam, but leaves the decision to Dazai. Meanwhile, Atsushi notices the prop arm in Naomi's possession, the same one that pushed him onto the fake detonation device. Bewildered, Atsushi is exhausted by the elaborate setup, all just to see if he qualified for the Agency.

In the end, Dazai points out that if Atsushi declines his invitation to the Agency, he will have no choice but to leave the Agency's dorms, which no place to go. Having no other alternative, Atsushi accepts the job.

Armed Detective Agency vs. Port Mafia Arc
In the Vortex Cafe, the young man from the entrance exam profusely apologizes to Atsushi for causing him so much stress. Kunikida tells Atsushi to not interfere nor besmirch the Agency's name, as they all take their roles as detective seriously (looking at Dazai, he leaves much to be desired). The young man introduces himself as Jun'ichirō Tanizaki, along with his younger, very affectionate sister Naomi.

Trying to get in the swing of things, Atsushi asks the members what they did before joining. Instead of answering directly, Dazai suggests making a game of guessing everyone's background, which Atsushi agrees to. He correctly assumes Tanizaki used to a student, Naomi currently one as well given her uniform. Kunikida refuses to play along, but Dazai outs him as being a former maths teacher, a past Kunikida detests recalling. Dazai encourages Atsushi to guess his former occupation, but Kunikida points out no one knows about Dazai's past, and there's even a monetary reward for guessing it. Amazed by the large sum, Atsushi is more than eager to guess, but after several attempts, Atsushi fails to guess correctly.

Tanizaki receives a call - a requested case by a young woman. Her issue is a group of dangerous men hanging around her company's building. After the explanation, Kunikida assigns the investigation as Atsushi's first mission, and orders Tanizaki to accompany him. As a result, Naomi joins as well.

Before Atsushi and the company depart, Kunikida warns him of Ryunosuke Akutagawa and the Port Mafia. Kunikida informs Atsushi that Akutagawaa has an ability solely for murder, and the Agency cannot handle him, thus combat against him is strictly discouraged. Kunikida himself knows he's no match for Akutagawa.

Later, led by the woman, Atsushi, Tanizaki, and Naomi find themselves in a secluded alley. Here, the woman reveals herself as a Port Mafia mafioso - one in direct contact with Akutagawa. She opens fire on the group, and the sight of Naomi's severe injuries brings Atsushi to his knees, at a loss for the situation.

His fear worsens after Akutagawa appears, having attacked Tanizaki when he wasn't looking. Akutagawa elaborates the Port Mafia's target is Atsushi himself, intending to claim an unknown third party's bounty on him alive, much to Atsushi's confusion. Akutagawa greatly disturbs Atsushi, telling him it's his fault that his comrades lay dying on the pavement, causing flashbacks to the orphanage headmaster's harsh words - both blame misfortune on Atsushi's existence, condemning him for even living.

After witnessing the ability Rashomon, Atsushi stumbles on the ground. Tanizaki, still conscious, urges him to run away. Instead, Atsushi's impulses drive him to narrowly avoid another attack, where he grabs Higuchi's gun to try and take Akutagawa down. Rashomon devours the bullets, making it impossible for Atsushi to defeat Akutagawa in this situation.

Rashomon devours Atsushi's right leg, leaving him screaming in agony. The trauma makes him relive another flashback of the headmaster's abuse. Here, the headmaster strikes and admonishes Atsushi for crying in pain, telling him no orphan abandoned by his own family has the right to cry. Although Atsushi knows he's been abandoned and mistreated his whole life, he acknowledges that he's never given up regardless of the fact.

The tiger takes over, regenerating Atsushi's leg, impressing even Akutagawa. Their battle rages on. When it appears Akutagawa kills him, it's nothing but one of Tanizaki's illusions. The real Atsushi attempts to ambush Akutagawa from behind, however, Dazai appears, nullifying both Beast Beneath the Moonlight and Rashomon. This renders Atsushi unconscious.

Atsushi later awakens in the detectives' agency's medical ward, with Kunikida seated beside him. After he remembers what happened he immediately inquires about the well being of Tanizaki and Naomi and is informed that they are alright. When the two begin discussing the bounty on Atsuchi's head, Kunikida explains that they will not waver and fight the mafia. However, Atsushi notices that Kunikida actually seems anxious and starts the feel guilty about putting the agency in danger. Then after asking Atsushi to think of how he can help, Kunikida leaves the room.

After hearing an explosion, Atsushi goes down the street to find the aftermath of an operation carried out by the mafia's combat division "Black Lizard" and worries about what would happen if they attacked the office. He then calls Higuchi using her business card and he tells her that he is quitting the Armed Detective Agency.

He later bumps into Kunikida when he is leaving the office with his belongings. But comes rushing back when he hears gunshots coming from the direction of the agency, only to find that the detectives had utterly trounced the mafia's "Black Lizard". Then when Doppo reveals that the "worst-case scenario" in this situation is needing to apologize to the neighbors, Atsushi begins to cry in relief realizing the Detective Agencies true strength and that he has found a place to belong.

While Atsushi helps clean the office with the rest of the agency he notices that Ranpo receives a ton of praise despite not helping and insulting the others. He is then asked to aid Ranpo on his next job, who states he doesn't need an assistant but help using the train.

The two of them later arrive at the murder scene and meet with officers Minoura and Sugimoto, the former stating they don't want their help. It is then explained that the victim, Minoura's subordinate Yamagiwa, was found in the river early in the morning with three bullet holes and no other evidence. Suddenly a body is caught in the police search net, but it turns out to be Dazai. Atsushi questions if Dazai was trying to commit suicide again, but he explains that he was not. Shortly after Ranpo teases Sugimoto trying to get him to solve the case in 60 seconds, Atsushi thinks to himself that he probably looks that flustered most the time. He is also unimpressed by Ranpo's motto of "If I'm fine, everything is fine." After Ranpo determines that Sugimoto is, in fact, the murderer using his "Super Deduction", Dazai pushes Atsushi towards the armed officer, causing him to miss his shot. Atsushi then manages to disarm him and pin Sugimoto to the ground.

Afterward, Atsushi is present at the police station when Sugimoto confesses to the murder, explaining that he was working for a Senator who ordered him to stop Yamagiwa from exposing his corruption. Ranpo then proceeds to guess to correctly guess Yamagiwa's last words causing Sugimoto to reveal the location of the corruption evidence. Later while leaving the station Atsushi is shocked when Dazai explains to him that Ranpo's "Super Deduction" is not actually an ability, he solves the crimes with his own intellect and this is why Ranpo is never admonished despite his attitude.