The Duke de La Vallière (Duke of the Vallière) is married to Karin, Duchess de La Vallière of Tristain. In his youth he served in the Tristain army alongside his future wife. Through an unknown ancestor he is related to the Tristain royal family.
He has three daughters; Éléonore, Cattleya, and Louise.
When the Duke first appears in the series he is shown complaining about Cardinal Mazarin and his support of the war. His disgruntlement comes from his belief that Tristain, despite receiving support from Germania, is still to weak to successfully challenge Albion. Due to this he refuses to support Louise's plan to join the military as he knows she will not survive. Later, he, his wife, and eldest daughter walk in on Louise and Saito kissing, an act that angers the Duke to such an extent that he orders for Louise to be lokced in a tower for a year and for Saito to be beheaded. The duo manage to escape though and escape their punishment.
Shortly after his daughter and her fellow students return from rescuing Tabitha from Gallia, his wife decides to confront their daughter and deal with her as punishment for her treason. On hearing this, the Duke begins to grow nervous as he, more than anyone else, knows how powerful Karin is. While he attempts to calm her, his words fall of deaf ears as she proceeds to equip her old arsenal.