Who is Isola Pribby from The Guernsey Literary and Potato Peel Pie Society and what is their importance?

Isola Pribby is a member of the Guernsey Literary and Potato Peel Pie Society, who befriends Juliet through letters. She is known as a witch because she sells potions at the market. When Sidney visits Guernsey, Isola acts as his hostess. Her study of Phrenology helps her stop Billee Bee from stealing her Granny Pheen's letters from Oscar Wilde.