Clarisse van Rossum
Clarisse van Rossum is one of the oldest, if not the oldest Magical Girl alive. Being born sometime in the 19th century (in the Netherlands, if her name is any indication) by now she's one of the few, if not the only, person past 500 years of age. She usually appears to be around 30 years old, though. Her hair is wavy and her face is freckled.
Van Rossum's chosen vocation is historian, with her wish implied to be in the line of "witness historical events". In this vein the extent of her power waxes and wanes with (physical and temporal) proximity to historical events. Furthermore it gives her a feeling as to where something significant will happen, or at least where she ought to be to witness it. As many other ancients she has spent time to expand her magical repertoire as well, giving her the ability of, among others, short range teleportation and telekinesis. These are coupled with the aforementioned proximity to events, though, making some only available on the cusp of these historical happenstances.
Furthermore van Rossum's power includes the ability to record powers and memories of willing or dead persons in a book, which she can call upon in certain cases and places, making her on those occasions a metaphorical Swiss Army Knife of Terror if she so desires.
It is heavily implied that she is the Magical Girl that originated Walpurgisnacht, e.g. in Chapter 22 Clarisse arrives at Mitakihara on the same place and time that Walpurgisnacht did when witches existed.