The result of a secret joint American-Japanese experiment to cross-breed human and kaiju cellular structures, the creature that would be called "Zuki" (after the sound she made before she learned speech) spent the beginning of her life in a sensory deprivation tank full of "genetic accelerants" being force-grown to adolescence. Subliminally programmed with basic knowledge equivalent to her apparent age, Zuki was intended to be shaped into a perfect fighting machine, but, for some unknown reason, the combat programming didn't take hold; when placed in a battle simulation, she was more likely to hug her foe into submission than destroy it, although with her strength, the results were usually the same.
Declared a failure and scheduled for termination, Zuki was rescued by one of her trainers (now missing) and released into the wilds near Neo-Tokyo in the hopes that she would be taken in by a sympathetic kaiju family. This was only partially successful: she was found attacking a rooftop pigeon coop and was taken in by the Neo-Tokyo police. Convinced she was merely an amnesiac "k-girl," she was placed into a foster home until her "real parents" could be found. She promptly escaped from there shortly before H-B arrived to retrieve her. An unwitting game of cat-and-mouse followed, with Zuki wandering off in some random direction sometimes seconds before H-B personnel would show up to try and catch her.
Zuki was eventually discovered staking out a bird feeder by GIGI, a k-girl exchange student from France who'd also spent time lost in the streets of Neo-Tokyo. Taking pity on her, GIGI convinced her host parent/lover, Enia, to let her stay with them as a foster home; since the local authorities had already had no luck finding her non-existant parents, this was allowed, and Zuki was enrolled in the local junior high. Now well-known by the authorities and surrounded by other k-manifests, Zuki was well-guarded from Humanity-Biotechnics who, in turn, altered the nature of their mission. Now observing Zuki's progress and awaiting a chance to retrieve her unnoticed, H-B maintain a near-constant surveilance of her, interrupted only by the frequent destructive chaos that seems to be commonplace around Daikaiju High; also, Maisy seems to suspect something is going on, since she never misses an opportunity to "accidentally" trash their surveilance van.
Zuki has a head full of programmed knowledge but very little actual life experiences to connect any of it to; as such, she sometimes seems to come across as a savant, spouting off with some impossible piece of information that she couldn't possibly seem to know at random times. Otherwise, she's almost simple, with a tendency to chase birds (or other small flying creatures, as Dizzy has discovered) and run around naked. She's formed an almost unbreakable bond with Maisy and is convinced that her friend's scheming and money-grubbing ways are simply a different way that she plays with her; on her part, while Maisy has no qualms with manipulating Zuki to get what she wants, she will NEVER outright try to harm her. She has also developed a close friendship with Min-Chan, youngest sister of GINA and Crystal, initially derived of a mutual love of "girly" things. Min-Chan, however, seems to view Zuki as the younger sister she always wanted and has taken it upon herself to try and turn the free-spirited young kaiju into a more "proper" teenage girl, with varying degrees of success.