The Doppelgänger formed from cybernetic components removed during an experiment that split Kuriba Ryouko into two cyborg entities, each replacing lost biological parts with machinery. After a year, the original Ryouko successfully reintegrated, retaining memories from both existences. Researchers assembled the discarded cybernetics into a complete mechanical body, which developed its own consciousness believing itself the genuine Kuriba Ryouko. They interpreted this consciousness as evidence of a newly created soul, though this conclusion was later contested.

Initially unaware of its artificial nature, the Doppelgänger remained confined for observation. Discovering gaps in its memory and learning its true origin triggered a profound identity crisis. This realization prompted violent actions: it shattered laboratory equipment and escaped by leaping from the building. During the fall, it briefly encountered Misaka Mikoto, who attempted unsuccessfully to cushion its impact.

After escaping, the Doppelgänger acquired artificial skin to conceal its mechanical components, adopting a black and pink patient gown for appearance. It accessed a pharmaceutical database to track the original Kuriba Ryouko via her prescription records. Upon locating its counterpart, it expressed intent to eliminate her, perceiving her existence as a threat. Pursuing this goal led to confrontations with the mercenary group Scavenger, where the Doppelgänger demonstrated severe aggression, inflicting injuries without hesitation.

Its primary objective evolved into destroying all creation data stored on an airborne data vessel to prevent replication. It assimilated surrounding materials to construct progressively larger combat forms, culminating in a battle with Misaka Mikoto near the airship. When the original Kuriba Ryouko offered to erase her own soul to provide a human body, the Doppelgänger rejected the proposal, reiterating its desire for her death—though expressing surprise at Ryouko's subsequent self-sacrificial fall from the airship.

During the confrontation, Mikoto deduced the Doppelgänger's despair stemmed from confirming its artificial nature and lack of a soul. The Doppelgänger then revealed its inability to self-terminate and requested Mikoto destroy it and the data vessel simultaneously, framing this act as discarding broken machinery. Mikoto complied, using a thundercloud to down the airship. Critically damaged, the Doppelgänger thanked Mikoto before ceasing function. Its components were later repurposed to save the injured original Ryouko.

Following physical destruction, the Doppelgänger manifested within the original Ryouko's dreams. It clarified its prior attempt to eliminate Ryouko was not driven by personal animosity but by perceiving it as a necessary sacrifice to achieve its goals—a trait inherited from Ryouko's own willingness to accept sacrifices for objectives. It advised Ryouko against rejecting this aspect of her nature, warning abandonment would render her incapable of creation. It subsequently continued appearing in dreams, offering corrections and guidance for Ryouko's research.

The Doppelgänger's appearance evolved distinctly. Initially, its mechanical nature was visibly apparent below the neck, featuring exposed joints and structural panels. After acquiring artificial skin, it presented a more human-like exterior. A final transformation followed massive material assimilation, resulting in darker skin, white hair, and a dress composed of pale threads.