Paul is Maureen's older brother, who lives on his grandfather's pony ranch on Chincoteague Island. He is determined and resourceful, setting his sights on capturing the elusive wild horse known as the Phantom. As he reaches an age where he is permitted to participate in the annual pony roundup, he works hard to prove his maturity to the adult riders.

Maureen is a young girl with a golden mane of hair, living on Chincoteague Island with her grandparents. She is strong, independent, and a capable horseback rider. Along with her brother, she takes on odd jobs, such as gathering oysters and catching crabs, to save enough money to buy the wild pony they spot on Assateague.

The Phantom is a legendary wild chestnut mare living on Assateague Island. She is famous for her speed, her copper coat, and a distinctive white marking on her shoulder that resembles a map of the United States. For two years, she has successfully evaded the men during the annual pony roundup, maintaining her absolute freedom.

Grandpa Beebe is a longtime resident of Chincoteague and a pony rancher who cares for his grandchildren while their parents are away. He is a gruff but deeply loving man with wiry whiskers and a wealth of local folklore. He serves as the primary supporter for Paul and Maureen's ambitions, offering them a stall for the horse they hope to catch.

Grandma Beebe is the warm, practical matriarch of the Beebe household. She has a round face and is constantly busy caring for Paul, Maureen, and Grandpa Beebe. She expresses her love through home-cooked meals, keeping the family fed and providing steady encouragement during stressful events.

Misty is the silvery-gold foal of the Phantom. Unlike the adult wild ponies, Misty displays an immediate comfort with humans and the domestic world. Her friendly disposition contrasts heavily with her mother's fierce independence, showing a natural affinity for life on Chincoteague Island.

The Pied Piper is the large, protective stallion who leads the wild herd on Assateague Island. With his wild, flowing mane, he aggressively defends his family and uses loud cries to guide the mares and foals away from danger.

The Assateague Ponies are the wild herd of horses that populate the barrier island. Descended from domestic horses that survived a historic Spanish galleon shipwreck, they have adapted to the harsh environment over generations, forming tight familial bands that graze on salt grass.

Wyle Maddox is the leader of the roundup team during Pony Penning Day. He directs the riders across the island and instructs Paul on his specific duties during his first year participating in the event.

Kim Horsepepper is a young boy who rides alongside Paul during the annual pony roundup on Assateague Island, aiding in driving the herd toward the channel.

The Fire Chief of Chincoteague oversees the annual pony sale, which serves as a fundraiser for the local fire department. He organizes the events and handles the transactions regarding the captured horses.

Foster is a man visiting the Pony Penning Day events who places an early down payment on the Phantom and Misty, temporarily thwarting Paul and Maureen's plans to purchase the horses.

Watch Eyes is a white-eyed pony belonging to Grandpa Beebe. Paul rides him during his first participation in the Assateague roundup, relying on the horse's stamina to help capture the wild herd.

Black Comet is a confident, well-trained horse who has won the annual Pony Penning Day race for three consecutive years. He is the prime competitor against whom the other racing horses are measured.