Penelope is a young girl that Stewie becomes infatuated with in "Mr. and Mrs. Stewie".

When Brian takes Stewie to the park to make up for a bad day, he meets Penelope, who extracts revenge on a boy who pushed Stewie at a park by giving him superglue-laced taffy. While playing at her house, Stewie discovers she has the same fascination for weapons as he does. Unlike Stewie, Penelope was able to kill her mother and together they cause chaos in several parts of the world. When Brian tells Stewie that she is becoming a bad influence, she asks him to kill Brian but Stewie is unable to do the job. She tries to kill Brian herself but Stewie intervenes and fights Penelope. When Stewie gets the upper hand she agrees to leave Brian alone. Penelope kisses Stewie goodbye before riding off with a smile.

Penelope is voiced by Cate Blanchett.