Longuevuille

Matilda de Saxe-Gotha (マチルダ・デ・ザクセン・ゴータ, Machiruda de Zakusen Gōta), known professionally in Tristain Academy of Magic as Longuevuille (ローングビル, Rōngubiru), is Osmond's former secretary. She is later revealed to be the thief Fouquet (フーケ, Fūke), a member of Reconquista.

Background
Matilda came from a noble kin, but, due to some reason, they lost their rank. Despite this, she is still capable of performing magic. At some point, she presumably adopted the name Longuevuille to work in Tristain Academy of Magic as Osmond's secretary. It is unknown from when she started working as a thief behind the headmaster's back, assuming the infamous identity of Fouquet.

Entrance Ceremony
On the first day of classes at Tristain Academy of Magic, Longuevuille levitates Osmond's smoking pipe, reminding him it is also her job to look out for his health. She also dismisses him after he is caught touching her backside; he immediately acts senile with this. He recalls that tomorrow shall be the Familiar Spirit Summoning Exam, where his familiar, Motsognir, is caught peeking at Longuevuille's undergarments, much to her surprise and anger. Afterwards, an explosion is heard from the other building caused by Louise de la Vallière's attempt to perform alchemy.

The Plebeian Familiar Spirit
Colbert consults Osmond about Saito Hiraga, especially the runes engraved on him. Surprised, Osmond apologetically dismisses Longuevuille; as she leaves, she catches a glimpse on Colbert's book, showing the runes that read Gandálfr. From afar, she glares from a window at Saito after his victory against Guiche de Gramont.

Post-Void Day
At the Academy, Longuevuille sees Count Mott after visiting the headmaster; the Count invites her to dinner sometime, but she declines. Longuevuille inquires about his visit, to which Osmond answers that they merely warned them about Fouquet the Clod, who has been actively lately, stealing treasures especially from nobles. He adds that the Palace wishes the Academy to be extra careful about the Staff of Destruction that is stored in the Academy Treasure Vault.

Fouquet Strikes
Longuevuille reports the last minute preparations to Osmond before the Familiar Exhibition the next day. He sneakily orders his familiar mouse like always to peek at her undergarments, but she catches the mouse and the headmaster. Later, Princess Henrietta arrives at the Academy, welcomed by the students and staff.

During the final preparations at night, Colbert assigns guards from the Vault to guard the gates. Longuevuille encounters him and shows worry on his decision, given that Fouquet is on the loose lately, particularly after the treasure in the Vault, but Colbert dismisses the idea, especially that the Royal Guard would be present should the thief opt to infiltrate the premises and because the entrance would be improbable to enter by a triangle-class mage.

After the incident of Fouquet's attack during the Familiar Exhibition, Longuevuille briefs Louise, Tabitha, Kirche, and Saito on the gathered information about the thief and shows them a sketch of Fouquet, further confirmed by Louise and Tabitha. Amidst suggestions to seek help from the Palace, Osmond decides to take the matters at hand, asking help from his fellow mages who wish to take the mission in catching the thief. However, not one from them are up for the challenge, except for Louise and, later, Kirche and Tabitha. Longuevuille volunteers to go with them as their guide.

As they head out in their mission, Longuevuille explains how some mages are not necessarily nobles, as some have been demoted to peasants, just like her case. Kirche asks her how she lost her rank, but Louise stops her behaviour. Eventually, Kirche gives Saito the shiny sword she had purchased for him. Later, they stop near a shack that is believed to be Fouquet's hideout. Upon preliminary inspection, there seems to be no traps, so they head inside to further check. Louise leaves herself behind to guard outside, while Longuevuille would scout the surrounding area. After the rest of her teammates defeat the golem using the Staff of Destruction, Longuevuille arrives and takes the Staff, revealing herself to be Fouquet all along. Fouquet points the Staff to them, but Saito thrusts the back of Derflinger on to her, knowing that the rocket launcher is a single-use weapon.