Mitraillette, whose code name is French for "submachine gun," is the elder daughter of the Thibaut family, the younger sister of Nazi collaborator Etienne Thibaut, and the second-in-command of Damask circuit.
She serves as most of Maddie's female companionship while Maddie's stranded in France, commiserating with her over what a letch Paul is and helping her out in any way she can—when Maddie falls apart after killing Julie, Mitraillette is the one who picks up the pieces, holding her and dosing her with knock-out medicine when she needs it.
Mitraillette is a take-charge kind of girl, always managing to keep Maddie safe when she's posing as Käthe Habicht or hiding out in the barn. When the ambush on the prisoner van falls apart, she makes the decisions and gets the survivors out of there: "Mitraillette thought of everything—perhaps she's done it before?" (2.18.12). Whether she has or not, it's clear there's good reason Mitraillette is second-in-command.