The Witches : What is Grandma?

She's not a witch, but she's definitely not a normal old lady. How did she loose that finger? How did she know so much about the witches? Her and the Grand High Witch have a history. In the one scene GHW says Grandma is old, "very old".

I think the implication is that she was a former victim/captive of the witches who survived her ordeal, albeit with one less finger. During her time as their captive she witnessed things in private that the witches do not reveal to the public, which is why she knows so much about them. Most other other victims don't escape.

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The sun is shining but the ice is slippery.

In the book she describes herself as a "witchophile," someone who studies and destroys witches. She also says that the story of how she lost her finger was too scary to tell, and the reader never does find out.

in the book she is a witch hunter

In The movie version grandma has information about witches and lost her friend to a witch. The movie also suggests that she and the grand high with have past history with her knowing she was a witch but not the grand high witch and being responsible for her missing finger . She and the GHW share a mutual recognition, but neither one acts like they know each other to their faces until the very end.

Helga is most probably the descendant of a White Witch, like the one Miss Irvine becomes at the end of the movie.