Victor "Granpapa" Thistle is a minor character in the series. He is Holly, Daisy & Poppy's grandfather (King Thistle's father).
Granpapa Thistle is, as most other characters have pointed out, "bonkers". He is just as heavy a magic user as his wife, Granny Thistle, but instead uses it mostly for his crazy hobbies and ideas.
He is always looking for the most ridiculous ways to deal with situations, such as escaping out of a volcano or venturing down the center of the Earth. He is sane but is also insane.
He cares much for his wife, and they both have much in common.
He, Granny Thistle, and the Wise Old Elf used to attend college together where it's implied that they were all very good friends in the past.