Lou, originally named Louise, is a human from Mirrorside Village transformed into the Living Doll for Shadow Louise through selection and brainwashing. Before this, she attended school with John, Patrick, and Shirley, known as a role model who frequently smiled and playfully teased Patrick. On the day before her selection, she persuaded Patrick to join her, John, and Shirley at the circus, where they aided an unnamed orphan (later Emilico) to participate in the selection under the alias "Kate".

Characterized by quiet obedience, Lou prioritizes her master's orders and her personal safety. She exhibits significant concern about potential facial damage, often invading others' personal space to inspect for imperfections. Though capable of determination, especially regarding Louise, she struggles with independent decision-making, viewing herself solely as a "face". Her appearance features wine-red curly bob hair, bright yellow eyes, and a Victorian-style maid dress with a black bow on her bangs. Before entering Shadows House, she wore a pale-yellowish dress with a frilly collar.

Shadow Louise appears as a featureless silhouette resembling Lou, dressed in a yellow Victorian-inspired dress with floral accessories and dark-blue flats. She possesses a sassy, confident, and narcissistic personality, often prioritizing fun over serious matters. Initially, she adores Lou solely for her physical appearance, frequently touching her face and leaving soot residue, which she hastily cleans. She proudly displays her "face" and keeps sketches of Lou in her room.

Their relationship begins as a clear master-servant dynamic, with Louise controlling Lou's expressions and movements. During the Debut Arc, they achieve first place in the rankings by perfectly synchronizing during Edward's examination and a subsequent dance. Louise's soot ability manipulates facial expressions via inhalation, initially used primarily on Lou to enforce loyalty and synchronization.

Development occurs as Louise gradually acknowledges Lou's humanity. After Lou rescues her from a pit trap during the garden maze trial, Louise expresses gratitude and adorns both their hairs with flowers. She later demonstrates concern for Lou's physical well-being, ensuring she rests and avoids exhaustion, though the power imbalance persists. Louise also uses her soot power to aid allies, controlling a Veiled Doll during Emilico's rescue operation.

Lou exhibits subtle autonomy and affection beyond her programming. She secretly gifts her shears to Ricky during the maze trial, expressing hope for both him and Patrick to pass their debut. Later, while acting as Louise's face, she requests a flower from Ricky, justifying breaking the rules by stating, "It’s because it’s Ricky," and glances back at him when leaving. Her past habit of leading Patrick by hand resurfaces as their suppressed memories fade.

Louise trains with Benjamin in the gym alongside their Living Dolls and respects his position as Star Bearers secretary. She collaborates with Kate and John, reporting Edward's misdeeds to the Star Bearers and participating in Maryrose's team meetings about proper conduct within the manor. In private moments, Louise listens to Lou sing, indicating shared intimacy beyond their formal roles.

Lou's name means "famed warrior" of German origin, while Louise is the feminine form of Louis, meaning "famous warrior" in French. Louise’s signature flower is the Gerbera daisy, and her soot emission patterns curve when she experiences negative emotions like anger or anxiety.