Who is Billee Bee from The Guernsey Literary and Potato Peel Pie Society and what is their importance?

Billee Bee is a temporary employer who is hired as Sidney's secretary, but she makes herself indispensable by offering to run errands for Sidney and Juliet constantly. She goes to Guernsey to collect Isola's letters written by Oscar Wilde, but she tries to steal them. Billee Bee is Gilly Gilbert's mistress who seeks revenge against Juliet and Sidney because she was embarrassed by the earlier incident between Juliet and Gilly.