Polly Merriman is the baby sister of Felicity Merriman.
Polly is technically mentioned in Felicity Saves the Day, when it is mentioned that Martha Merriman is due to have a baby in the winter. She is born during the events of Felicity's New Sister, but in book release order she first appears in Changes for Felicity. She is named Polly as she was almost born in the water like a polliwog.
Polly is an infant, and thus has a minimal personality.
In the Books
Polly is born in the abandoned house that the traveling group takes shelter in when the carriage is broken on the way to the plantation. Rose says that she was almost born in the stream like a pollywog, and so Felicity suggests that they name her Polly.
Changes for Felicity: A Winter Story
In the Film
Polly's birth is announced by Nan, who says that they have a new sister when Felicity arrives home. Her birth causes Mother to fall extremely ill, and thus unable to complete Felicity's gown. Felicity and Rose mind her while her mother is getting better.
She is played by an uncredited Genevieve Harvey.
As a part of Felicity's Collection
Polly is, uniquely, the only sibling of a historical character to be given a doll in any American Girl collection.