Atsushi Nakajima, an orphan expelled from his institution at eighteen ostensibly due to funding shortages, was actually cast out because his uncontrollable nightly transformations into a destructive white tiger devastated crops and storehouses. Before his expulsion, he endured severe physical and psychological abuse from the orphanage director, surviving being chained without food and torture like a nail hammered into his foot. Other orphans frequently scapegoated him, deepening his social isolation and entrenched belief in his own worthlessness. Profound self-doubt and a persistent sense of being cursed define his personality, yet he instinctively acts with selflessness. Intending to steal while homeless, he instead saved Osamu Dazai from suicide and later risked his life shielding strangers during a simulated bomb threat—his entrance exam for the Armed Detective Agency. He constantly questions if his actions stem from selfishness, believing his value depends solely on protecting others, evident when rescuing train passengers while wondering if it justifies his existence. His empathy extends even to adversaries; he grieved upon learning of his abusive orphanage director's death, revealing a complex capacity for forgiveness beneath his hatred. He forms a protective bond with Kyoka Izumi, a former assassin, viewing her as someone he must save from a dark past. This dynamic reinforces his conviction that saving others gives his life meaning. However, he struggles with paralyzing fear when facing threats alone, initially panicking confronting the terrorist Kamui. His confidence surges significantly when fighting alongside allies, particularly during his reluctant partnership with Ryūnosuke Akutagawa. His ability, "Beast Beneath the Moonlight," allows transformation into a white tiger under moonlight, granting enhanced strength, speed, regeneration, and senses. Initially uncontrollable, he masters partial and full transformations after joining the Armed Detective Agency, aided by Yukichi Fukuzawa’s ability, "All Men Are Equal." Later, he temporarily merges his power with Akutagawa’s "Rashōmon," gaining elongated claws and a black coat. Across his journey, he evolves from a traumatized outcast into a resolute defender of Yokohama. Key developments include his alliance with Akutagawa defeating Guild leader Francis Scott Key Fitzgerald and stopping the airship Moby Dick, confronting the orphanage’s legacy during the director’s death, and thwarting global threats like the Decay of the Angel and the vampire outbreak. Akutagawa’s sacrifice during the Kamui conflict profoundly impacts him, reinforcing his resolve despite enduring trauma. Recurring flashbacks to the orphanage gradually shift from sources of debilitation to reminders of his resilience.