Louise Françoise Le Blanc de La Vallière is the main heroine of The Familiar of Zero. She is the third daughter of the Vallière family, a distinguished noble household that owns the north-eastern territory of Tristain. She is a sixteen-year-old second-year student at the Tristain Academy of Magic, where she is known by the cruel nickname Louise the Zero because every spell she attempts fails spectacularly, typically resulting in a massive explosion, and she shows no affinity for any of the four standard elemental magic systems. She has long pink hair and is small in stature.

Her personality is defined by an intense pride in her noble lineage and a fierce, hot-tempered nature. She is a classic tsundere character: outwardly arrogant, quick to anger, and prone to berating those around her, yet beneath this exterior she harbors deep insecurities about her inability to perform magic. She desperately craves acceptance and respect from her peers and family. Her motivations stem from a desire to prove that she is not worthless and to live up to the expectations of her prestigious name. She is fundamentally a hardworking and determined individual who refuses to give up despite constant failure.

Louise serves as the central female protagonist. Her role in the story begins when she accidentally summons Saito Hiraga, an ordinary Japanese high school boy, as her familiar during the academy's springtime summoning ritual. This event sets the entire narrative in motion. Initially, she treats Saito poorly, using him as a servant, but their relationship slowly deepens into a close bond and eventually romantic love. A major turning point in her role is the discovery that her magic is not a failure but instead belongs to the lost and legendary fifth element known as Void magic, making her one of the only four Void mages in the world of Halkeginia. She becomes a key figure in protecting Tristain and its royal family, particularly her childhood friend Princess Henrietta.

Key relationships drive her character arc. Her bond with her familiar Saito is the most important, evolving from master-servant disdain to mutual reliance and genuine romantic affection, culminating in their marriage at the series' end. She maintains a loyal friendship with Princess Henrietta, for whom she would risk her life. She has a bitter, long-standing rivalry with the flirtatious and well-endowed Germanian classmate Kirche von Zerbst, which stems from historical feuds between their families as well as personal jealousy. She also develops relationships with other classmates such as the stoic mage Tabasa and the womanizing Guiche de Gramont. Within her own family, she holds her strict mother Karin in awe and fear, and she deeply adores her kind older sister Cattleya.

Throughout the series, Louise undergoes significant personal development. She overcomes her initial feelings of inadequacy and learns to control her powerful Void magic, transforming from a mocked student into a confident and capable mage. Her emotional growth is closely tied to her relationship with Saito, as she learns to lower her defenses, express her feelings, and trust others. She matures from a spoiled and impulsive noble girl into a responsible young woman willing to fight for her friends and her kingdom.

Her most notable ability is Void magic, a rare and immensely powerful system that is distinct from the four common elements. Her known Void spells include Dispel, which can negate other magic, and the destructive Explosion spell. She later masters a spell called World Door, which allows her to open portals between dimensions, enabling travel between her world and Saito's original world of Earth. Her magical power is innate and grows as she gains confidence and training. Aside from magic, she is a skilled horsewoman and enjoys activities like knitting, though she is not particularly good at it. She also fiercely detests frogs and has a deep fear of her mother's disapproval.