Fouquet

Fouquet (フーケ, Fūke) is a member of Reconquista and a triangle mage mercenary, who uses a giant golem made of earth to steal magical valuables; signature to her, she would leave messages whenever she completes a robbery. Her alter ego was Longuevuille (ローングビル, Rōngubiru), an identity she uses as a secretary of Osmond in Tristain Academy of Magic.

Background
Matilda hails from a noble family, but, for some reason, they lost their rank. Despite this, she can still perform magic, excelling until the level of a triangle. At some point, she assumed the identity of Fouquet and became an infamous criminal in Tristain. She presumably adopted the name Longuevuille to work in Tristain Academy of Magic as Osmond's secretary, though it is unknown when she started to work behind the headmaster's back.

Post-Void Day
Count Mott, a Palace messenger, comes to the Academy to warn them of the recent activity of Fouquet in stealing treasures around the country.

Fouquet Strikes
In the middle of the night, Fouquet infiltrates a house to steal some jewels, leaving behind a message roughly translating to: "The treasure that was stored in your house has definitely been transferred into my hands. Fouquet the Clod"

On the day of the Familiar Exhibition, Fouquet infiltrates the Academy Vault, but notes that it is difficult to open. Instead, she summons a golem and tries to force it open through a series of shockwaves, but to no avail. Louise and Saito arrive at the scene and face the golem. Saito tells Louise to flee, but Louise attempts to set it on fire, though only managing an explosion. The explosion causes the building to crack, leading Fouquet to have her golem punch it. With the building collapsing, the rest of the students are alerted. Tabitha, riding Sylphid, surveys the area; Fouquet manages to steal the Staff of Destruction and escapes. Before stealing it, she manages to leave a message: "The Staff of Destruction has been transferred into my hands. Fouquet the Clod"

After the incident, Osmond sends Louise, Tabitha, Kirche, Saito, and Longuevuille to capture Fouquet. During the mission, Fouquet's golem appears and fights the group. After Saito uses the Staff of Destruction — a rocket launcher — to defeat the golem, 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.

Aftermath
Fouquet is then taken by the Royal Guard into custody and the Staff of Destruction is secured in the Academy Vault. One night, Fouquet is visited by an unknown assailant who breaks her out of captivity.

Academy Summer Vacation
Fouquet and her masked accomplice visit Cromwell, ordering them to proceed with their plan.

Trivia

 * Fouquet is likely named after Nicolas Fouquet, a corrupt government official under Cardinal Mazarin and an underage Louis the XIV, who most famously accumulated a large fortune while serving as finance minister until his eventual arrest in 1661.
 * In the light novels, Fouquet was stated as part of high-class Albion nobility.