Possibly Charlotte (whose backstory IS detailed in the art books), Gertrud (no real known backstory, but an amazing and very firm theory as to her identity) or H.N. Elly/Kirsten (no official backstory, but the art book helps with hinting at the girl she used to be).

Basically, Witches are born from wishes turned inside-out and so they curse people, and you can see traits from who they used to be, but they're, well. Twisted. Each Witch is a witch of something and has a specific Nature that is based on who they used to be at their core.