Who is the magical girl who becomes Walpurgisnacht?
There is no information about who it was that became Walpurgisnacht. All we really know is that Walpurgisnacht is, in some sense, a fusion of multiple witches (or magical girls, I guess).
Official description from July 2011 Megami interview with Urobuchi:

In the last episode we saw the &quot;Walpurgisnacht&quot;, but what kind of witch is it specifically?
It has the destructive power to bring about natural disasters powerful enough to blow away an entire town, but originally it was a single witch. It's a witch that has grown from the combination of countless other witches. Walpurgisnacht combines with other witches in the same way two powerful tornadoes are able to combine and become larger. It's essentially a &quot;conglomeration&quot;-type witch. Because it's so powerful, it rarely shows itself.

See the Madoka wiki for more details.