Iris is a twelve-year-old who is profoundly Deaf and lives in Houston. She attends a mainstream school called Timber Oaks where she is the only Deaf student, leaving her frequently isolated from her hearing peers. To cope, she spends her free time repairing antique electronics, feeling their vibrations to ensure they work. When she learns about a unique whale singing at a frequency no other whale can hear, she becomes determined to compose a song to reach him.

Blue 55 is a hybrid whale, half blue whale and half fin whale. He sings at an unusual 55-hertz frequency, making his calls completely unintelligible to other whales who sing at lower pitches. Because he cannot successfully communicate even with his own parents, other pods continually abandon him. His unanswered songs and isolated existence mirror Iris's own struggles with communication.

Iris's maternal grandfather was a vibrant member of the Deaf community who met his wife in a Deaf theater group. He shared a strong bond with Iris, acting as her primary connection to Deaf culture and her trusted mentor during trips to local junkyards. His passing leaves a massive void in Iris's life, though his memory and creative teachings continue to guide her actions.

Iris's maternal grandmother is a Deaf woman who was once bold, dynamic, and outgoing. Following the death of her husband, she retreats into a solitary state of mourning. When Iris approaches her with a wild plan to locate Blue 55, the prospect of traveling to Alaska provides Grandma with an opportunity to escape the retirement home and reclaim her former vitality.

Iris's mother is a hearing woman who grew up as the child of two Deaf parents. Because she felt excluded from her parents' tight-knit Deaf community as a child, she now fears losing Iris to that same culture. This anxiety causes her to insist Iris attend a mainstream neighborhood school rather than the specialized academy Iris desperately wants to attend.

Iris's father is a hearing man who describes himself as a numbers person rather than a words person. He has a poor grasp of sign language, which creates an ongoing communication barrier between him and his daughter. Despite this deficiency, he tries to bond with Iris by sharing his childhood fascination with humpback whale songs and space exploration.

Iris's older brother is a teenager who drives his own truck. He has a strong command of sign language and often acts as a mediator between Iris and their parents. While he is generally supportive of his sister, he fails to grasp the emotional depth of her fixation on the lonely whale.

Wendell is Iris's close friend who is also Deaf. Unlike Iris, he comes from a hearing family that has made the effort to learn fluent ASL. He attends Bridgewood, a school with a strong Deaf community, giving him a social advantage that Iris acutely lacks.

Bennie is a girl from Northern Canada traveling on the cruise ship where her mother works as a naturalist. She is curious and open-minded, taking an immediate interest in Iris's mission to find Blue 55. Bennie actively learns sign language to communicate better with her new friend.

Andi is a sanctuary staffer based in Appleton, Alaska, who previously attempted to tag Blue 55. She corresponds with Iris about the whale's habits and migration patterns. While she encourages Iris's acoustic project, she views the girl's desire to meet the whale in person as impractical.

Ms. Conn is a teacher at Timber Oaks who handles Iris's deafness with impatience and insensitivity. She fails to understand ASL structure, dismissing Iris's handshape poem because it lacks verbal rhyme, and frequently attempts to force Iris to lipread rather than use an interpreter.

Ms. Alamilla is Iris's science teacher. She introduces the class to the concept of hertz and tells the story of Blue 55, inadvertently sparking Iris's obsession with the lonely whale. She maintains a warm, encouraging presence in Iris's academic life.

Mr. Charles is Iris's ASL interpreter at Timber Oaks school. He acts as the essential bridge between Iris and her hearing teachers, sharing a quiet, respectful rapport with his student. He often suppresses amusement at Iris's signed witticisms during class.

Nina is an over-eager classmate at Timber Oaks who attempts to communicate with Iris using patchy, nonsensical sign language. Her fast, erratic hand gestures frustrate Iris and often draw unwanted attention from hearing peers, making Iris appear uncooperative when she fails to understand.

Mr. Gunnar owns the local antique shop where Iris sources parts for her electronics. He provides Iris with broken radios to fix, paying her for her successful repairs and treating her with professional respect. He communicates with her by facing her directly and writing notes.

Sura is the cruise ship's naturalist and Bennie's mother. She gives lectures on marine life, specifically explaining whale behaviors like bubble-net feeding, and helps Iris track Blue 55's migration patterns during the voyage.

Mr. Russell is the school music teacher. When Iris presents him with the unconventional sheet music for Blue 55, he enthusiastically gathers student musicians to help record the unique 55-hertz frequency for the lonely whale.