Upon meeting detective Osamu Dazai, Atsushi learns he has a supernatural power called "Beast Beneath the Moonlight" that allows him to turn into a large white tiger with incredible strength, speed, durability, and regenerative capabilities.

However, writers appreciated his rivalry with Port Mafia's Ryūnosuke Akutagawa and his care of former assassin Kyoka Izumi; they felt Atsushi became stronger because of these relationships, which formed his character arc.

Rather than adding gore to Atsushi's fights with his rival Ryūnosuke Akutagawa, Igarashi showed the characters in silhouette in front of a red background.

[9] When reading the manga for the first time, anime director Yasuomi Umetsu was impressed by how detective Osamu Dazai mentored Atsushi and Akutagawa.

[11][12] The theme song used in the second season of the anime, "Reason Living" by Screen Mode, focuses on Atsushi's attempts to become a stronger person.

[13] For the 2018 film Bungo Stray Dogs: Dead Apple, Asagiri told fans to focus on the psychological development of Atsushi, Kyoka Izumi, and Akutagawa.

Asagiri said that throughout the film, Atsushi stays true to his original persona of a weak and relatable person, despite having scenes showing his inner strength.

According to the voice actor, a major impact on Atsushi's development is how he finds Kyoka, wishes to protect her from a dark fate, and becomes a stronger person for doing so.

Despite having difficulty in showing the character's strength, Uemura believed some lines Atsushi has when interacting with people from the Guild gave a good impression of his power.

[19] In the making of the film Dead Apple, Uemura felt honored to keep playing Atsushi, mainly due to the character's importance and his impact on his career.

During his search for Kyoka, Atsushi clashes with Akutagawa to stop the airship Moby Dick from falling on the city, and the two encounter Fitzgerald.

[35] While preparing to face Akutagawa, Atsushi is attacked by Nikolai Gogol, one of Fyodor's partners from the terrorist organization Decay of Angels, and is wounded.

[37] Atsushi appears in the film Bungo Stray Dogs: Dead Apple, where the detective agency learns of a series of suicides by people who possess supernatural powers.

Atsushi embraces his power, which he calls Byakko and will-to-fight, and once again kills Shibusawa, with Kyoka and Akutagawa's help, which results in the fog disappearing.

[45] In promoting the film Dead Apple, Tobu Zoo—in Miyashiro, Saitama Prefecture—displayed a picture of Atsushi alongside the tiger mascot "Rocky-kun".

During the promotion of this event, Atsushi's voice actor, Yūto Uemura, participated in making multiple announcements that were run between February 24 and March 3, 2018.

Upon his introduction into the manga series, The Fandom Post said that his ability to become a tiger offered him the power to do good rather than making him feel cursed.

[54] Anime News Network felt that, while initially weak, Atsushi could become, little-by-little, a stronger person, due to the way Dazai guides him.

[56] Anime News Network deemed Atsushi charming when he was protective of Kyoka Izumi in the beginning, saw parallels between the two characters based on their traumatic pasts, and described the bond they have when interacting as strong.

[59] While analyzing the story, Gamerant Saw that both Atsushi and Kyoka shared the saddest episode in the entire series for how they deal with parenting issues, though the former's case was harder to put into words as a result of the caretaker's constant abuse on the protagonist and how this shaped his growth and trauma.

Otaku USA stated that Atsushi's insecurities contrasted with Akutagawa's tendency to be violent in a mad quest to gain the respect of his peers.

The exploration of the interactions between these two characters was commented to improve both of their characterizations during the anime's second-season climax,[61] where both Atsushi and Akutagawa come to accept their differences and make a temporary peace in order to join forces to defeat Fitzgerald and the Guild.

The Fandom Post appreciated how the rivalry progressed across the story and felt that the final battle was enjoyable, with Fitzgerald showing signs of humanity.

With the villains being regarded as weak, Atsushi's central conflict was said to have a major impact on the plot due to his realization of having killed one of the orphanage's abusers in a fit of rage, which suppressed the trauma.

Once Atsushi accepted being the tiger, Anime News Network felt that he participated, alongside Akutagawa, in one of the most interesting fight scenes.

[69] Retorno Anime stated that Atsushi's past is one of the most interesting parts of the film, which extends the original work while adding further depth.