Polly Merriman is the baby sister of Felicity Merriman.
Polly is described as a rosy baby with sky-blue eyes and hair as orange as carrots. Polly was often a nickname for Mary or Dorothea during the era, referencing the respective nicknames Molly and Dolly. However, similar to the future first lady Dolley Madison, it is quite likely that Polly is her full first name, as she was so named by Felicity after Rose says she was almost born in the water like a polliwog.
Polly is first mentioned technically in Felicity Saves the Day, when it is revealed that Martha Merriman is due to have a baby come the winter of 1775. She is born during the events of Felicity's New Sister and in the Central Series first appears roughly a month after her birth in Changes for Felicity.
In the Books
Mother's pregnancy is brought up by Mrs. Wentworth when she discusses Lord Dunmore and his family leaving the colony and that Martha should be glad to be away from Williamsburg in her condition given the Patriot actions happening in Williamsburg. Mother blushes as it is something of an improper topic to discuss at a meal table in the era.
Polly is born in the abandoned house that the traveling group takes shelter in when the carriage is broken on the way to King's Creek 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
Polly is seen a month after her birth as Felicity comes in the house to announce that Penny is due to have a foal. Nan is protecting her face from the fire's heat with a fire screen, and William is amusing her with a jumping jack toy. Mother states, upon hearing about the upcoming foal, that Felicity will do well with them as she has done a fine job helping to care for Polly.
The day that the children must stay indoors, Felicity cares for Polly and amuses her with a spinning top until Grandfather's return. She also keeps Polly while Grandfather is ill. After his death, holding Polly reminds her of Grandfather's words that nothing is sweeter than a baby and happier than a new life starting.
Martha is shown pregnant, unlike in the books. 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 similar to the events of Felicity's Surprise, and thus unable to complete Felicity's gown. Felicity and Rose frequently 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; she is in the item Baby Polly and Cradle.