Louise Françoise Le Blanc de La Vallière is the third daughter of the Vallière family, an aristocratic family well known in Tristain. She is attending classes in the Tristain Academy of Magic, where she is known as Louise the Zero due to her zero success rate at spell casting and lack of any elemental affinities, a nickname acquired even before the first episode.
As a Noble, Louise defends her honor as a benevolent and "superior" person, like other Nobles. She states that is her responsibility as a noble, to make sure her highness the princess is safe and that her familiar's well being. Louise is a very stubborn girl. But, as Henrietta mentions, her heart is pure. Louise is also very persistent.
Her poor magical ability is due to her alignment to void magic, an ancient branch of magic which only four people can currently use, with the the last known user before them being Brimir; because of the branch's rarity the Academy does not have the ability to properly train Louise in its use, unlike other students who can learn their magical specialization from teachers who share their alignment. Void magic itself seems to be mostly capable of only destruction; any time she tries to use it for anything else it will result in an explosion of varying magnitude. She has been known - however - to be able to do relatively simple spells, but her use of void magic requires a certain amount of time to build up before use.
MMPH! MMPH! (Translation: Let me outt this!!)