Yukie worked as a housekeeper in the mansion shared by Misa and Masato, unaware of their immortality. With a warm, cheerful demeanor, she cared for Masato as his nanny, forming a bond he later exploited. Her life brightened upon becoming engaged, prompting her to resign and pursue a hopeful future. During her final days, she confided her plans to Misa, unknowingly provoking Masato—emotionally dependent on her—to gift her mermaid flesh. Driven by a desire to replace Misa as his eternal surrogate mother, he sought to bind Yukie to an immortal cycle he had sustained for centuries.

Consuming the flesh cursed her instead, warping her into a violent Lost Soul that turned on both immortals. The traveler Yuta intervened, destroying the creature to prevent further harm. Amid the aftermath, Yukie’s shattered engagement ring was found, a silent testament to her abruptly severed dreams.

Though Yukie had no prior ties to mermaid legends, her seemingly ordinary, trusting dynamic with Masato masked his manipulation. Her rapid transformation from optimistic caretaker to tragic monstrosity highlights the mermaid flesh’s volatile power, its corrupting influence upending lives without warning.