Abducted from Mongolia as a child by the criminal organization Inferno, an ordinary girl had her memories erased. Subjected to intensive psychological conditioning and combat training under Scythe Master, she emerged as Inferno's deadliest assassin, codenamed Ein—German for "one," marking her as Scythe Master's first perfected weapon.

Operating under the alias Phantom, Ein executed high-profile assassinations with ruthless efficiency, showcasing exceptional skill with firearms including the Colt Python and H&K MK23. Conditioned to suppress emotions, she saw herself as a phantom devoid of past or purpose, convinced that shedding emotions was vital for survival. She instilled this philosophy in Zwei—the protagonist, abducted and renamed—teaching him to abandon his identity and conscience.

Her evolution started with Zwei. Initially detached, Ein slowly bonded with him during shared missions. Branded a traitor due to an Inferno power struggle, Zwei's decision to shield her rekindled her ability to trust. In their ensuing escape attempt, Zwei's use of her true name "Elen" symbolized her reconnection to humanity, but their recapture led to Scythe Master forcibly resetting her memories.

Later, Ein's buried emotions reemerged. Freed again from Scythe Master's control by Zwei, she pursued her roots, recalling her Mongolian heritage. This journey climaxed in Mongolia, where she reclaimed her identity and felt true emotional release—even smiling for the first time.

Her story concludes differently across adaptations. In the anime, Ein and Zwei gained freedom, but Zwei was soon assassinated; she then ingested poisonous petals to die beside him. Visual novel endings vary: one has her succumbing to wounds after killing Scythe Master; another features her living incognito with Zwei; a third sees her sacrificing her life for Zwei's escape; and a fourth portrays her reclaiming independence and carrying on alone after Zwei's death.