Grantz is Baron Wulfenbach's monster hunter; she hunts monsters for him when they cause trouble. While this generally implies dealing with things such as rogue machines and beasts, it can also include people; for instance, she has indicated that she considers Bangladesh DuPree to be in this category.
She appears to have unnatural strength; aside from wielding a sword far beyond normal size, she is able to carry an Imobilex Jug in one arm, something Sergeant Scorp notes that he would likely require a heavy caisson to assist with.
Back when he was dueling clanks as part of his grieving Agatha's death, Gil made reference to a male Grantz when he asked Wooster about a newly-brought in rogue clank. This may be a spouse or relative of this Grantz, or it may be (this being a comic about Mad Science) that our Grantz has, for one reason or another, switched genders in the last few weeks; it is also possible, given Grantz' generally masculine appearance and Gil's long absence in Paris, that Gil might simply have been unaware of her true gender. Alternatively, it may just be a typo on the Foglios' part.
Curiously, her massive sword (not only is it a two-handed sword with a blade approximately 1.5m-long, it has no fuller and has parallel edges at least 5cm apart almost all the way to the point) has a thick crescent cross-guard that appears to flip. The guard curves toward the point when the sword is, but toward the pommel when sheathed. However, since the sword has been seen unsheathed on only one page so far, it is possible that this is not indicative of some hidden mechanism, but rather merely a slight artistic slip in one panel.