*might contain spoilers*
Atsushi Nakajima is a 18 year old boy who was thrown out of his orphanage, now he is a member of Armed Detective Agency. His ability is Beast Beneath The Moonlight which allows him to transform his body parts into one of a white tiger's. When Atsushi was thrown out of his orphanage, he met Dazai who took him into Armed Detective Agency, but before that Dazai made him realise that he was thrown out of the orphanage because, he is a white tiger with whom the orphange was troubled. Atsushi didn't know about his ability to turn into a white tiger.
Atsushi had a great inferiority complex, due to the way he was mistreated in the orphanage during his childhood. He is weak and he lacks in confidence, his wants to know the purpose of him being alive or is he even worthy of being alive. But also he is over protective, selfless and pure at heart who would do anything to protect someone else, even if he doesn't know them. He reacts negatively to those who judge others as unworthy.
Atsushi was not able to control his ability neither he knew how to use it, but when he comes to agency under the influence of Fukuzawa's (President of the agency) ability 'All Men Are Equal', he was able to control it with his will. After coming to the agency, he came to know that someone in the black market had put a bounty of seven billion on his head and now The Port Mafia was after him. In his initial days, Atsushi was very low at self-esteem and at self- worth, he doubted his abilities and even questioned his existence, during a fight, he freezed due to fear, his muscles tensed and he was not able to do anything, but gradually he strengthened after having multiple fights with Akutagawa and the guild, he started gathering courage and became reliable. As much as he took the initiative to save Kyoka, who was an ex-assassin of The Port Mafia and Lucy, from the guild, they both look up to him as an inspiration.
When Atsushi stoped questioning his self- worth, he found someone like him, Akutagawa, though he doesn't think of Akutagawa as someone similar to him. Actually they are rivals, and Akutagawa wants to kill him for good maybe because Dazai approved of Atsushi, but they worked together on most of the cases and were successful. Dazai thinks that they both are the only onews who can surpass The Double Black, the duo name given to Dazai and Chuuya.
Atsushi's character has developed the most during the tenure of the show. Though Atsushi is not very comical, but he is the most "normal"person in the Armed Detective Agency who shows some common sense.