"People need to be told they’re worthy of being alive by someone else or they can’t go on"
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. This is seen through his interactions with Kyōka and Lucy, 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 can relate to those 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.
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.
- (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!"
- 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 anime 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.
- 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.