Annie is a 20-year-old Irish American woman from Boston and a recent graduate of the Perkins School for the Blind. Having experienced visual impairment herself, she wears smoked glasses to protect her light-sensitive eyes. Haunted by the childhood death of her brother at a state almshouse, she brings a fiercely independent, unsentimental approach to teaching that heavily contrasts with the Southern customs of her new employers.

Helen is a six-and-a-half-year-old girl living in Tuscumbia, Alabama, who has been deaf and blind since infancy. Clever and energetic, she acts entirely on instinct and mimicry, ruling her household through violent tantrums. She possesses an intense desire to communicate and understand the world, though her family's pity prevents her from learning boundaries.

Kate is the second wife of Captain Keller and Helen's devoted, optimistic mother. Exhausted by the daily chaos, she serves as Helen's primary protector and frequently uses sweets to stop her tantrums. She drives the search for a teacher, holding onto a fragile hope that someone can reach her daughter's mind.

Captain Keller is a proud Southern gentleman, newspaper publisher, and Civil War veteran. He is deeply traditional and increasingly exhausted by the chaos his daughter brings to his home. Having largely given up hope for Helen's recovery, he acts highly skeptical of the young Northern teacher sent to help them.

James is Captain Keller's indolent eldest son from a previous marriage. He struggles to adjust to his father's new wife and harbors significant resentment toward his family situation. Sarcastic and cynical, he views Helen as a lost cause and frequently mocks Annie's efforts to control her.

Aunt Ev is Captain Keller's sister and a frequent presence at Ivy Green. She embodies Southern tradition and family pride, believing strongly in the inherent resilience of a Keller. She spoils Helen while pushing the family to consult experts for a cure.

Viney is a servant in the Keller household who manages daily operations and assists with the children. Like the rest of the adults, she sneaks Helen sweets to avoid tantrums and carefully navigates the tense dynamics of the Keller family.

Dr. Anagnos is the director at the Perkins School for the Blind in Boston. He acts as a steady mentor to Annie, handling her stubbornness with patience while securing her a job to prevent her from returning to the state almshouse.

Percy is Viney's son and a frequent playmate for Helen. Familiar with Helen's chaotic ways, he eventually serves as an assistant to Annie when she relocates to the garden house, acting as a buffer to help Helen transition into a new routine.

Martha is Viney's daughter who spends time around the Keller household. She frequently endures Helen's aggressive physical curiosity during their unstructured playtime.

Mildred is the Kellers' infant daughter. Too young to speak or walk, she inadvertently becomes a target of Helen's jealousy and lack of understanding regarding boundaries in the house.

Jimmie is Annie Sullivan's deceased younger brother. He died when they were children at the state almshouse, and his memory deeply haunts Annie, fueling her strict exterior and her reluctance to love anyone again.

Belle is the Kellers' family dog. She is a docile presence in the home and occasionally serves as an unwitting participant in Helen's early attempts to mimic Annie's hand signs.