Kate, a faceless Shadow noble inhabiting a grand mansion, possesses an entirely black form and wavy hair styled in two ponytails secured by rose-shaped barrettes. She typically wears red clothing with white accents, including a dress, vest, and Mary Jane shoes. Unlike other Shadows, Kate originates from outside the Shadows House; her true identity is Kate Mirror, heir to the overthrown Mirrors House, the mansion's original owners. Born to Catherine Mirror after a prolonged fifty-year pregnancy caused by her mother's transformation into a Shadow, Kate was raised in exile by her mother's maid, Emily, posing as her grandmother. Initially human, Kate gradually transformed into a Shadow due to her mother's condition, leading her to isolate herself before fleeing to the Shadows House at age thirteen to uncover her family's history.

To infiltrate the mansion, Kate disguised herself as a Morph—a soot creature—using a black cloth obtained from a nameless girl (later revealed as Emilico). Captured and brought inside, she observed Morphs in the Mimicry Room and mimicked Emilico's appearance to blend in, eventually becoming Emilico's designated Shadow master.

Initially reserved, distant, and easily angered, particularly toward Emilico due to the latter's brainwashing, Kate's early interactions involved curtness and frustration over Emilico's clumsiness. She demonstrated underlying care by sharing high-quality food and teaching Emilico to read. Emilico's unwavering optimism and kindness gradually softened Kate, leading her to become more open and protective. Kate's inability to express emotions visibly contrasts with her internal stoicism and occasional intense anger, manifesting as violent soot emissions.

A key development is Kate's discovery of unique soot manipulation abilities. She learns conscious control, moving objects like Emilico's doll Rolly and projecting soot from her hands. This rare power among Shadows becomes central to her strategies and is kept secret. Her intellect and calmness under pressure aid in solving mansion mysteries, positioning her as a strategic thinker.

Kate's relationships drive her growth. She views Emilico as a trusted partner and friend rather than a servant, naming her to emphasize individuality. Their bond evolves into deep interdependence, with Emilico acting as Kate's emotional anchor. She maintains a pragmatic alliance with John, who harbors unreciprocated romantic feelings, and a respectful but cautious dynamic with Louise and Patrick. She holds strong enmity toward Edward, who targets her after her debut, and respects Barbara while criticizing her methods. Her connection to Shirley, who communicates through a ribbon after reverting to a Morph, provides crucial insider knowledge.

Motivated by her heritage and the Shadows House's injustices, Kate organizes a rebellion against Lord Grandfather, the ruling Shadow responsible for brainwashing inhabitants via soot-laced coffee. She recruits allies in the children's wing, including John, Louise, Patrick, and their Living Dolls, to expose system cruelties—such as the fate of "unified" Living Dolls who lose their identities. Her leadership focuses on gathering information, delaying confrontations until prepared, and protecting Emilico from threats like kidnapping and brainwashing.

Kate's backstory intertwines with others in the "Flashback Arc," revealing she met Emilico briefly before entering Shadows House. During this encounter, Emilico (then a nameless circus performer) gave Kate the cloth used for her disguise, and Kate later inspired Emilico's name. This history underscores their fated connection and Kate's role as a catalyst for Emilico's journey.