Jill is a fifth grader and the first-person narrator of Blubber. Her best friend, Tracy Wu, describes Jill as “really tough on people” (1). Jill’s mom also describes her as a “pretty tough character sometimes” (168). Jill is a very picky eater, often opting for simple peanut butter sandwiches over anything else. When Great Maudie moves in and replaces the food in the house with healthy food, Jill believes she “might starve to death” (146). Likely due to her picky eating, Jill is underweight. The school nurse asks Jill to try to gain five pounds by “drinking a malted every day” (93). Jill is also a chronic nail-biter, but at the beginning of the book, she relays a deal she’s made with her dad. If she can grow her nails until Christmas, he'll give her $25 to spend in the stamp department at Gimbels. Jill is proud of her stamp collection and is a prideful person in general. Her bitterness for not having won the most original costume at the Halloween event at school further demonstrates this trait. Jill believes her costume was the best and deserved to win.

Jill has a skewed sense of right and wrong throughout the book.