Atsushi Nakajima (中島 敦, Nakajima Atsushi?) is the main protagonist of the Bungo Stray Dogs manga and anime series. He is a member of the Armed Detective Agency who possesses the ability Beast Beneath the Moonlight.
Atsushi has light gray hair, pale skin, and is of average height with a slim build. He has segmental heterochromia, consisting of purple with yellow along the bottom curve of his pupil. His hair is short and swept forward in loose spikes with a lock left long on the right side of his face. This uneven style is the result of bullying he suffered at the hands of other orphans in the orphanage, and he never fixed it.  Additionally, he has a prominent black streak in his hair.
Initially, he wears rags issued by the orphanage. After he joins the Armed Detective Agency, Akiko Yosano and Doppo Kunikida issue him a white button-up with the sleeves pushed to his elbows, a loose black tie, black pants, black, fingerless gloves, and a suspenders/belt combo with a long stretch of the belt dangling loosely from the buckle.
When he transforms into a partial tiger, his eyes become tiger-eye like and he develops fangs. If more drastic, his limbs morph into tiger limbs, and his tail emerges.
Atsushi is enthusiastic and passionate, caring deeply about his fellow members of the Armed Detective Agency and for the well-being of civilians and of people in bad situations. During his entrance exam that all members must pass to join the Agency, he is faced with a simulated bomb threat. He uses his body to cover the bomb and shield the other members, whom he had never met.
His traumatic past affects much of his daily life, and he is tormented with nightmares, flashbacks, and the internal monologue of the orphanage director who abused him. He has a severely low sense of self-worth and is somewhat naive, which causes him to initially feel helpless when confronted with dangerous situations. When paralyzed in guilt and/or fear, he has difficulty reacting or understanding what to do. He also tends to react strongly and believe things at face value, trusting others' judgment above his own, even abandoning his own instincts.
For example, when learning of the seven billion yen bounty on his head, he becomes convinced that he is putting the Agency at risk. He feels intense guilt and decides to leave, believing it will keep the Agency from being targeted. He is advised by Kunikida to learn how to become useful rather than worry about the Agency being attacked.
He has a strong sense of morality, which also directly stems from his upbringing. He believes his worth in life depends on his ability to protect others and has an aversion to the abuse of one's power and to preventable deaths. He values life and tries to help even his enemies at times. This is seen through his interactions with Kyōka, Lucy, and Sigma and during his battles with Akutagawa. However, on the flip side, Atsushi has a black/white binary view of morality and of good/evil of the world in which he tries to help those he can relate to who have suffered like him, but ignores and rejects the personal struggles or suffering of those he deems "evil", even when his opponents can teach him greater lessons about combat, life, or different world views. This is most evident through his view of Akutagawa whom he refuses to understand and whom he has hypocritically dehumanized as a monster who can only cause destruction and kill, despite Akutagawa's many attempts to understand Atsushi as a person. This quality of Atsushi has greatly hurt his partnership with Akutagawa in battle as he had no trust in him due to his rigid views of him in his mind.
Although still struggling with his self-esteem and the words of his late Headmaster, Atsushi gradually improves his confidence and sense of self after joining the Armed Detective agency. He continues to be strengthened by several missions and the clashes between the Port Mafia, Guild, Rats in the House of the Dead, and Hunting Dogs (among others). His will hardens in these tough situations, and he becomes steadfast, reliable, and intensely caring. However, he still experiences paralyzing fear of fighting alone and often needs a companion to aid him in battle or at times save him in order to feel powerful.
Atushi 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' abuse, Atsushi suffered at the hands of the other orphans. Oftentimes, 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 years old, 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.
Beast Beneath the Moonlight (月下獣, Gekka-jū?): Atsushi has the ability to transform into a white tiger. At first, he doesn't seem to have much control over the transformation, likely due to the fact, or caused by, his unawareness of his state as an Ability user.
After becoming an official member of the Agency and under the influence of All Men Are Equal, he gains more control over his transformations - as shown during his second fight with Akutagawa, when he breaks the transformation on his arms to escape.
|Atsushi Nakajima vs. Ryunosuke Akutagawa (alley)||Interrupted (by Dazai)|
|Atsushi Nakajima vs. Kyōka Izumi|
|Atsushi Nakajima vs. Ryunosuke Akutagawa (ship)||Win|
|Atsushi Nakajima vs. Kōyō Ozaki||Lose|
|Atsushi Nakajima and Ryunosuke Akutagawa vs. Francis Scott Key Fitzgerald||Win|
|Ryunosuke Akutagawa and Atsushi Nakajima vs. Alexander Pushkin||Interrupted (by Goncharov)|
|Ryunosuke Akutagawa and Atsushi Nakajima vs. Ivan Goncharov||Win|
- (To Osamu Dazai) "At the orphanage, people always told me I'm worthless. I have no idea where my next meal will come from or my next bed. No one would care if I died on the street. I might as well just let that beast eat me and be done with it."
- "Suddenly, an idea came to me. A foolish idea that I can't get out of my mind. If I have any chance of saving them all, of returning them home safely, would that mean it's okay for me to keep on living?"
- (To Ryunosuke Akutagawa about Kyōka Izumi) "It's not up to you to decide who's worth living. Why couldn't you have spoken to her some other way? People can't live unless someone tells them 'it's okay to go on!' Why can't you even understand something that simple!?"
- (To Lucy Maud Montgomery) "You've got the wrong idea. I'm not strong, and I'm not popular. In fact, my entire life has been cursed. I know full well how you feel, envying and hating everyone around you." 
- (To Ryunosuke Akutagawa) "It's true that I'm foolish and worthless. But it sure beats being you - a murderer who just wants to bandy his power around! Am I wrong? You've got the power and status but you still fight 'cos you want to be feared! That's far more worthless in my book." 
- (Referring to the tiger) "No matter where I run, the tiger follows. It's the same as the way I can't escape the beating of my own heart… for you're my strength to keep living. I hear you well, now. I understand your words well. I know. I know everyone's souls are on fire. I'll say the same to you. Hurry up, or I'm leaving you behind. Come, tiger!"
- (While facing Kamui) "I don't fear the enemy. I don't fear the pain. What is fear is that I'm alone. […] Solitude scares me. At the orphanage, solitude was the norm for me. But now, this same solitude scares me. I wish there was someone fighting at my side. Anyone would do. Somebody…"
- (After escaping from Ōchi Fukuchi) "Why did Akutagawa let me go? I don't know… […] Moreover, was there some sort of important meaning in that? Or did it end that way by sheer coincidence? I didn't understand."
- He and several other characters made appearances in mobile games, such as Love Heaven, Kimito Lead Puzzle 18, and Yumeiro Cast as limited-time characters.
- When Atsushi Nakajima is wearing his belt, it seems to act like his tail as if he were in tiger form. When Atsushi Nakajima is relaxed, the belt hangs down. If Atsushi Nakajima is surprised or scared the belt acts much more lively.
- In BONES' anime adaptation, the black streak in his hair is omitted, yet it still appears in promotional art and the mobile game.
- According to the first official guidebook: 
- He thinks that his weakness is that he is weak and he doesn't think he has strengths.
- His motto is "It's okay as long as I'm alive."
- His ideal type is a kind person.
- His favorite spot in Yokohama is the shopping district.
- According to the second official guidebook: 
- He recently spends his free time borrowing books from the library and studying.
- As a habit from the time he lived in the orphanage, he goes to sleep at 9 and wakes up at 5.
- During his pair-up with Akutagawa, Atsushi felt that they weren't paired up in his heart.