Atsushi has light gray hair, pale skin, and is of average height with a slim build. He has segmental heterochromia, consisting of purple on the top half of his iris and 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.
Before joining the agency, he wears rags issued by the orphanage , and after he joins the Armed Detective Agency, they issue him a white button-up with the sleeves pushed to his elbows from Kunikida, a loose black tie from Yosano, black pants from Fukuzawa, black fingerless gloves from Ranpo, suspenders from Kenji, and a long black belt from Dazai.
When he transforms into a partial tiger, his eyes become tiger-eye like and he develops fangs. If more drastic, his limbs turn into into a tigers, and his tail emerges. 
Atsushi during the agency's cannibalism aftermath celebration
Atsushi wearing a cap
Atsushi disguised as a ship worker
Wan! appearance - preschool Atsushi
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.
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. 
Atsushi 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 orphanage director, Atsushi's parents dumped him. The director 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 director emphasized Atsushi's hatred towards him, seemingly even encouraging it. Alongside the director 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?) gives 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 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."
The name Atsushi (敦) is a Jinmeiyō kanji which are kanji that can be used as personal names.
The surname Nakajima means "inside" (中) (naka) and "island" (島) (shima/jima).
Although not politically active, Nakajima is known for his refusal to write propaganda stories for the Japanese government as so many of his peers did. The ability "Beast Beneath the Moonlight" references the short story "Tiger-Poet", about a man who transforms into a tiger because of his internal conflict between written and oral communication. 
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.
Ryunosuke Akutagawa's and Ichiyō Higuchi's nickname for Atsushi, "Mantiger", may be a reference to the alternate title of "The Moon Over the Mountain", The Tiger Poet (人虎,,Jinko?, lit. Weretiger).