Éléonore Albertine le Blanc de la Blois de la Vallière is the eldest daughter of Duke Sandorion de la Vallière and Karin de la Vallière, and older sister to Cattleya and Louise de la Vallière. Inheriting her father's stern and arrogant nature, she exhibits a quick temper, a domineering attitude, and impulsive actions. Éléonore consistently belittles Louise's magical talent, dismissively calling her "Chibi-Louise" and reprimanding her frequently. Both sisters share a propensity for physical aggression when angered, with Éléonore notably wielding a whip.

She possesses blonde hair, amber or pink eyes, and wears glasses, carrying herself with aristocratic poise. A noblewoman of the distinguished Vallière family in Tristain, Éléonore is a powerful triangle-class earth mage, ranking her among Halkeginia's stronger magic users.

Her personal history includes a canceled engagement to the Count of Burgundy, leaving her sensitive about marriage matters and motivating her pressure on Louise to marry quickly. Like Cattleya, Éléonore previously worked at Oriz Academy of Magic and later served temporarily as a professor at Tristain Academy of Magic by Princess Henrietta de Tristain's order, teaching practical magic application.

Éléonore disapproves of Saito Hiraga, Louise's commoner familiar, telling him he has no right to call her "sister," a disdain rooted in his non-noble status and her low regard for Louise's judgment. Despite this antagonism and her general harshness towards the couple, she ultimately attends Louise and Saito's wedding alongside her family.

Throughout her appearances, Éléonore maintains her strict, arrogant demeanor while revealing complex motivations beneath her abrasive exterior. Her actions reflect a commitment to upholding noble traditions and standards, particularly concerning magical proficiency and social hierarchy, alongside her professional competence as a teacher.