Eleanor Albertine Le Blanc de la Blois de la Vallière is the eldest daughter of the prestigious Vallière ducal family and the older sister of Louise Françoise. She is significantly older than Louise, with an age gap of at least eleven years, positioning her almost as an aunt-like figure to her youngest sibling. Physically, Eleanor stands out among her sisters as the only one with blonde hair, a trait inherited from her father.

As a researcher at the Tristain Academy of Magic's Royal Magic Research Institute, Eleanor holds a respectable position, though her practical duties often involve work like creating magical images rather than more dangerous fieldwork. Her personality mirrors that of her strong-willed mother, Karine, as she possesses a pride that is even more intense and pronounced than Louise's, combined with a naturally high-handed attitude that leads her to look down upon others.

Eleanor's most notable personality trait is her vehement opposition to marriage, which she famously refers to as the grave of life. This stance stems from a past broken engagement; her fiancé, the Count of Burgundy, called off their arrangement, unable to tolerate her unique and demanding personality. The trauma from this incident caused her to give up on the idea of marriage altogether, and she becomes extremely agitated whenever the topic is mentioned in her presence, often advising others against it.

In her interactions with her younger sister Louise, Eleanor has a characteristic habit of pinching Louise's cheeks whenever they meet, always addressing her as little Louise. While this appears harsh on the surface, it originates from a protective instinct. When they were younger, Eleanor would intervene and discipline Louise herself to spare her sister from facing the even more severe reprimands of their formidable mother. This demonstrates that despite her stern exterior, Eleanor genuinely cherishes her youngest sibling. This love is further evidenced when she rushes to forcibly bring Louise home upon learning of her intention to join the war in Albion, wanting to prevent her from entering such a dangerous and seemingly meaningless conflict.

Eleanor holds an extremely rigid view of social class and status, which makes her initially oppose Louise's relationship with Saito, a commoner. The idea of a ducal daughter marrying a non-noble causes her such distress that she is said to have coughed up blood at the news. She is also deeply sensitive about her modest chest size, which has become a source of significant trauma for her as she approaches thirty with little chance of change. Despite her strictness, she is not inherently bad-natured, as she eventually comes to acknowledge Saito and Louise's relationship after seeing her sister's unyielding resolve.

A somewhat cowardly side of Eleanor emerges when she learns of the continent-wide crisis caused by the overload of wind stones, prompting her to quickly pack her belongings and flee to the relative safety of Saito and Louise's home. Her courage, however, surfaces when her loved ones are in danger; she joins Professor Colbert on the airship Ostland to rescue Saito from the elves, proving her loyalty through action. In these instances, she forms an unlikely partnership with the noble Malicorne, with whom she shares a combative yet cooperative dynamic.

Eleanor's character arc culminates at Louise's wedding, where she is deeply moved seeing her younger sister marry before her, lamenting her own inability to find a suitable partner. In a moment of open-ended possibility, Malicorne catches the wedding bouquet, and Eleanor reacts by stomping on it, a scene that hints at a potential future connection between them.