Mila is a teenager who has spent the majority of her life raised by a pod of dolphins after surviving a plane crash as a toddler. Found off the coast of the Florida Keys, she is brought to a research facility in Boston against her will. She possesses an innate love for the ocean, finding deep comfort in water and music. As doctors attempt to study her and teach her English, she increasingly notices the restrictive, isolated nature of human society.

Dr. Beck is a psychologist and lead researcher who studies children found in feral conditions. She oversees the development of both Mila and Shay, establishing a live-in research house to observe them constantly. She is highly ambitious, hoping to use Mila to learn how to communicate with dolphins to secure her own academic legacy. Her scientific curiosity often overrides her basic empathy for the children in her care.

Sandy is a research assistant working under Dr. Beck at the Boston facility. She acts as a maternal figure to Mila, spending significant time comforting her, teaching her English, and providing her with books and music. Unlike her superior, Sandy displays genuine empathy for the girls in their care and attempts to make their lives more comfortable.

Justin is Dr. Beck's teenage son who lives at the research house alongside the medical staff and patients. He recognizes his mother's controlling tendencies and empathizes with Mila's desire for freedom. He gradually builds a quiet, accepting friendship with Mila, attempting to bring her small comforts that remind her of the ocean, such as a radio and trips to hear traffic that sounds like waves.

Shay is a young child who was kept locked in a dark room for the first years of her life before arriving at the research facility. She speaks very little, often hopping instead of walking, and withdraws from the clinical demands of the researchers. She responds best to Mila, who approaches her with joy and acceptance rather than tests and measurements.

Mr. Aradondo is the custodian at the house where the research team and patients live. He is a Spanish-speaking older man who is generally fearful of the feral children residing in the building. His presence unintentionally sparks Mila's fragmented early childhood memories.

Dr. Troy is a medical professional at the research facility who eventually takes over the primary care and study of Shay. He attempts to teach Shay basic skills but struggles to facilitate the same progress that Mila achieves under Dr. Beck.

Dr. Peach is Mila's music therapist at the research facility. They provide Mila with instruments, encouraging her profound connection to musical expression as a way of bridging her ocean past with human communication.

Monica Stone is part of the emergency rescue crew that captures Mila from her island. She initially mistakes the swimming teenager for a mermaid. She earns a small amount of Mila's trust by offering her fresh drinking water from her hands during the capture.

A member of the dolphin pod who found Mila drifting alone in the sea after losing her own newborn calf. She adopted Mila, nursing and raising her with the rest of the pod. She provides Mila with a deep sense of family, protection, and belonging.