Born in Sereni's Far North, Nanachi endured childhood hardship as an orphan, scavenging discarded texts which cultivated literacy, especially in ancient glyphs. This existence ended when the White Whistle Cave Raider Bondrewd deceptively recruited Nanachi, transporting them into the Abyss. During the descent to the sixth layer, Nanachi formed a deep bond with Mitty, another orphan. Bondrewd subjected them both to an experiment using a device that transferred the Abyss's sixth-layer curse effects. The process concentrated physical and mental deformation onto Mitty, transforming her into an immortal, grotesque being devoid of higher consciousness. Nanachi retained humanoid form, personality, and intellect but gained a beast-like appearance featuring fluffy fur, long rabbit-like ears, and altered physiology.

Horrified by Mitty's suffering under Bondrewd's ongoing experiments, Nanachi orchestrated an escape from Idofront, relocating to a secluded hut on the Abyss's fourth layer. Driven by profound survivor's guilt and responsibility, Nanachi dedicated existence to caring for Mitty and researching methods to end her perpetual agony. This isolation fostered extensive expertise in Abyss ecology, medicinal plants, surgical techniques, and avoiding curse manifestations, alongside a reserved, often sarcastic or cynical demeanor masking deep compassion and emotional vulnerability, particularly concerning Reg.

Nanachi's solitude ended upon encountering Riko and Reg after Riko suffered severe Orb Piercer poisoning. Recognizing Reg's unique Incinerator as a potential means to permanently end Mitty's existence, Nanachi saved Riko's life through critical medical intervention and negotiated with Reg to use the weapon on Mitty. While fulfilling the goal to alleviate Mitty's suffering, this act precipitated intense grief and a loss of purpose, compounded by survivor's guilt. Reg's stipulation that Nanachi must not commit suicide forced a reconsideration of existence. Finding new direction, Nanachi joined Riko and Reg on their journey deeper into the Abyss.

Within the sixth layer's Golden City of the Scorching Sun (Ilblu Village), Nanachi encountered the Hollow Belaf, who possessed a replicated Mitty created from consumed memories of the original. Overwhelmed by renewed guilt and a compulsive need to atone, Nanachi traded their own freedom to Belaf for this replicated Mitty. This act led to confinement within Belaf's domain, temporarily separating Nanachi from Riko and Reg. The subsequent destruction of Ilblu Village by Faputa, the Princess of the Hollows, facilitated Nanachi's release. Following these events, Nanachi continued the journey alongside Riko, Reg, and Faputa towards the Abyss's lowest depths.

Nanachi utilizes the casual first-person pronoun "oira," exhibiting moments of shyness around Reg and a generally pragmatic, sometimes boyish speech pattern. The creator intentionally leaves Nanachi's gender ambiguous.