Who is Sheila Tubman from Fudge-a-Mania and what is their importance?

Sheila Tubman is a sixth grader girl and Peter's classmate. She is also Peter's arch enemy. Sheila is afraid of dogs, even though she has a little puppy. The Tubmans and Peter's family share a vacation house in Maine for three weeks. While there, she babysits for Fudge. Sheila is described as a know-it-all.