The narrator is a physically active boy growing up on a rural North Carolina farm in the early twentieth century. He values physical abilities like running, climbing, swimming, and rowing. He desires a brother who can share these athletic pursuits and feels deep shame regarding his sibling's physical limitations. He actively tries to teach his brother how to walk and engage in typical boyhood activities. His actions are driven by a strong sense of personal pride and a fear of social judgment from the surrounding community.

William is the younger brother of the narrator, born with physical limitations that lead his family to anticipate his early death. Despite these early struggles, he learns to crawl backward and eventually stands. He develops a deep appreciation for the beauty of Old Woman Swamp and often cries at the loveliness of the natural environment. He invents elaborate stories about magical people wearing golden robes and resting alongside peacocks. He deeply desires his older brother's approval and constantly seeks his companionship.

Aunt Nicey is a highly religious woman who lives with the family and assists in William's delivery. She interprets signs in the natural world through a religious perspective, viewing an intact amniotic sac as a holy omen that signifies potential sainthood. She remains the sole family member who maintains hope for the sickly infant during his early months. She frequently warns the family about bad luck associated with dead birds and other natural phenomena.

Mama lives on a rural North Carolina farm with her husband and two sons during the World War I era. She worries extensively about her younger son's health and cautions her older son about his brother's physical fragility, warning that the boy must avoid getting too hot or tired. She holds cautious views regarding sickness and the natural world, demanding the boys avoid touching a dead scarlet ibis in their yard. She tracks the war through the names of distant battles.

Daddy manages the family farm through difficult weather events like hurricanes and crop blights. He takes practical, unsentimental steps in response to his younger son's initial poor health, hiring a carpenter to build a tiny casket. He later constructs a go-cart so the older boy can pull the younger sibling around the property and out to the swamp. He expresses frustration at farm problems by cursing the weather and political parties.