RyuZU appears to be in her mid-teens, with long, flowing silver hair and a delicate, cherubic face. Her complexion is pale, contrasted by crimson lips and striking golden eyes. Naoto Miura describes her features as possessing an almost otherworldly beauty, enhanced by her smooth hair, fair skin, and slender frame. A seal engraved on the nape of her neck bears the inscription “Y. [R y u Z U].” Despite her full-sized form, her body is unusually light for an automaton. Her voice is likened to the clear, mechanical tones of a music box.

She is typically dressed in an elegant, old-fashioned black and purple gown, paired with a hair ornament crafted from white gears and a multicolored cloth of red, green, and black. Though often described as “so black it seemed tailored from the darkness of the night,” the dress predominantly features shades of purple and lavender. Concealed within her back are two scythe-like weapons made of gears, which can be deployed during combat or for collection purposes.

When activating her inherent ability, her attire transforms into a form-fitting, crystal-colored bridal veil, and her golden eyes shift into a vivid ruby red. On one occasion, while shopping for clothing, she wore a white camisole of velvety texture, a layered pink skirt, and a matching shawl, giving her an appearance reminiscent of a carefully wrapped confection.

As her designation suggests, RyuZU was created to serve her master, and in Naoto Miura’s case, she demonstrates unwavering loyalty. She is fiercely protective of him, prepared to use lethal force if necessary. Against humans, however, she often employs her abilities as a deterrent rather than escalating into violence. This restraint does not extend to other automata or individuals who attempt to harm Naoto or her sisters, as seen in her confrontation with Gennai.

RyuZU displays a possessive streak, showing jealousy whenever Naoto directs attention toward other automata or mechanical creations. On occasion, she has resorted to punishing him for perceived impropriety such as striking him during the destruction of Tokyo Tower and even allowed Marie Bell Breguet to stomp on him when his behavior turned lecherous. Her devotion also extends to safeguarding Naoto’s reputation, stating that his public image reflects directly on her as his servant.

Lacking an offensive language filter, RyuZU frequently speaks with sharp bluntness, often insulting others without realizing the severity of her words. At the same time, she elevates her own status, praising herself in the process.

Unlike many automata, RyuZU possesses free will and expresses a genuine desire to remain by her master’s side. She also takes pleasure in receiving recognition, occasionally requesting praise outright.

Advanced Automata Design
RyuZU was engineered by Y as the first unit of the Initial-Y series, making her far more advanced than standard automata. Unlike ordinary models, she experiences no startup delay when activating her Accelerator Gears. Her Shock Removers allow her to leap from heights such as a seventh-story building while carrying a human without injury to either party. Her Main Cylinder has been noted to pulse with a rhythm resembling a human heartbeat, and her spring is self-winding, enabling perpetual operation unless she sustains mechanical damage or activates her special ability, Mute Scream.

Heightened Perception
Among automata, RyuZU possesses exceptional cognitive abilities. She is capable of noticing fine details others overlook, such as discerning that Naoto’s school principal wore a wig. Her heightened awareness extends to overhearing covert conversations, such as Houka Hoshinomiya’s contemplation of orchestrating Naoto’s death, even while distracted. She can also process vast amounts of information quickly, demonstrated when she read through stacks of magazines and manga in the time it took for Naoto to shower.

Scythes
RyuZU houses two black scythes within her back that can be deployed during combat. These weapons are capable of cutting through most materials, though they are ineffective against extremely durable armor such as that of the Yatsukahagi.

Mute Scream
RyuZU’s unique ability, Mute Scream, enables her to shift from real time into “imaginary time,” creating the illusion of moving at extreme speed. While active, her perception of time accelerates dramatically, allowing her to defeat enemies in what appears to onlookers as a single instant. However, the ability cannot be deactivated at will and continues until her energy is completely depleted, leaving her immobilized afterward. Against her sisters, activating Mute Scream first would allow her to neutralize them before they could trigger their own unique abilities.

Weaknesses
Despite her advanced design, RyuZU is considered the weakest in direct combat among the Initial-Y series. If confronted by her sisters while they are using their inherent abilities, she would likely lose in a head-to-head confrontation. In addition, Mute Scream requires her to defeat all enemies within its range before the effect ends. If she fails, she is left vulnerable and unable to move until her systems reset.

Financial Skill
Beyond combat, RyuZU demonstrates exceptional management abilities. She has successfully increased Naoto’s finances through their activities, ensuring the group’s stability and supplementing Marie’s role as their primary source of support.

Naoto Miura
Naoto Miura becomes RyuZU’s master after reactivating her following more than two centuries of dormancy. Immediately after her repair, she saves him from a collapsing apartment and has remained devoted to him since, consistently addressing him as “Master Naoto.” RyuZU views his reputation as a reflection of her own standing as his servant, and she takes deliberate steps to improve his status following her reactivation.

Although she initially belittles Naoto, comparing him to a creature of little refinement, RyuZU acknowledges his extraordinary skill as a clocksmith, considering him the most talented in the world. At the same time, she frequently calls attention to his lack of social grace and education, often criticizing his behavior as indecent or perverted. While she finds it unsettling when Naoto occasionally contradicts her, she nonetheless respects his abilities and adapts to his responses.

Marie Bell Breguet
RyuZU’s relationship with Marie begins with a degree of disdain, as she openly admits she does not hold Marie in high regard. Despite this outward dismissiveness, RyuZU demonstrates an ability to fairly assess Marie’s talents, entrusting her with difficult responsibilities. On several occasions, she leaves Marie to handle crises that force the young engineer to her limits, reflecting both a severe standard of judgment and strong faith in her competence. RyuZU’s trust in Marie is especially evident when she relies on her to repair AnchoR in Thailand and later acknowledges Marie as her only chance of survival in the confrontation with TemP.

AnchoR
RyuZU holds deep affection for her youngest sister, AnchoR, and is overjoyed upon reuniting with her in Mie. Despite this bond, her devotion to Naoto remains absolute; she openly threatens to destroy AnchoR if she continues to harm him. During Naoto’s attempted kidnapping, RyuZU and AnchoR even contemplated a desperate strategy in which AnchoR would sacrifice herself to create an opening for RyuZU to escape with Naoto. After discovering that AnchoR’s hostility was caused by mind control rather than genuine intent, RyuZU rescues her and reaffirms her faith in her. Her respect for AnchoR is so strong that when her sister pleaded for TemP’s life, RyuZU honored her request.

TemP
RyuZU’s relationship with her elder sister TemP is marked by both love and enmity. She admits to loving TemP as family but is disillusioned by what she perceives as TemP’s abandonment of her intended purpose and abilities. To RyuZU, true love meant ending TemP’s existence to release her from what she considered a life of failure. Over two centuries before the story, she carried out this act, destroying TemP, and upon their reunion she resolved to do so again. Her animosity reaches a peak when TemP kisses Naoto out of spite, prompting RyuZU to attack her in a blind rage and declare her intent to destroy TemP while preparing for a lovers’ suicide with Naoto.

The name RyuZU (リューズ, Ryūzu) directly references the Japanese word for a watch crown, the small knob on a mechanical timepiece used to wind the spring and set the hands. This connection reflects her nature as an automaton and her association with clockwork mechanisms. The stylized use of uppercase and lowercase letters in her designation, “[R y u Z U],” is also engraved on the seal at the nape of her neck, serving as both an identifier and symbolic mark of her series.