Mia is a 20-year-old college student who has returned home to quarantine during the COVID-19 pandemic. A computer music major with perfect pitch, she approaches the world through a highly analytical, philosophical lens and often relates everyday sounds to musical notes. She struggles with a personal bias that equates linguistic fluency with intelligence, which complicates her understanding of her nonverbal brother.

John is Mia's twin brother, possessing a highly optimistic and caring demeanor that contrasts directly with his sister's cynicism. He works at Henry's House, a therapy center for neurodiverse individuals, and is deeply passionate about special education and disability advocacy. He shares a close, almost telepathic connection with Mia when they attempt to solve problems together.

Eugene is Mia and John's 14-year-old brother, who has a dual diagnosis of autism and mosaic Angelman syndrome. Because he cannot speak and experiences motor difficulties, he copes with stress through rhythmic jumping and vocalizations. His family's lives orbit around his care and their ongoing efforts to secure effective communication methods for him.

Hannah is an immigrant from Korea with a PhD in applied linguistics. As the primary earner for the family, she is fiercely protective of Eugene and carries intense guilt over her perceived inability to communicate with him. She frequently deals with xenophobic bias from people who underestimate her intelligence based on her immigrant accent.

Adam is the 50-year-old patriarch of the family who quit his job to become Eugene's primary stay-at-home caregiver. He is highly philosophical, deeply interested in the study of happiness, and meticulously records data in his "Happiness Quotient" notebooks. His sudden disappearance in the park sets the events of the story in motion.

Detective Janus is a six-foot-tall police officer with very short red hair who leads the investigation into Adam's disappearance. She operates with a persistent, nodding demeanor that Mia compares to a bobblehead doll. She is quick to interpret Eugene's physical actions as aggressive threats, leading to immediate tension with the family.

Shannon is the trusted lawyer for families attending Henry's House. Despite her doughy cheeks, wrinkled skirt suits, and comforting pancake-syrup scent, she is a highly competent and fiercely objective attorney. She prioritizes building a strong legal defense for Eugene above all other family concerns.

Anjeli is an energetic communication therapist known as a "nonspeaker whisperer" for her work with nonverbal individuals. She conducts private, text-based communication therapy sessions with Eugene using a letterboard. Her methods focus heavily on treating her patients as intellectually competent and capable of high-level thought.

Vic is Mia's ex-boyfriend from college, whom she broke up with during the pandemic quarantine. He is thoughtful and protective, driving eight hours across state lines to help search for Adam despite Mia having blocked his number. As a Black man, he carefully handles encounters with law enforcement and biased strangers in the neighborhood.

Harmonee is Mia's late grandmother who lived with the family in Korea. She survived extreme trauma during the Korean War and maintained a "pessimistic kind of optimism" that she passed down to Mia. She taught the twins a paradoxical practice of "whispered screaming" before Eugene was born.

Susan is an acquaintance of the Parkson family and a friend of the "Groupie Moms" at Henry's House. She encounters John and Mia at the park and mistakenly assumes they already know about police activity related to their father.

Kenny is a friend of John's who contacts him during the initial panic. He provides the family with a screenshot from a neighbor's "mommy blogger" video, alerting them to external documentation of Adam and Eugene's interactions on the day Adam disappeared.

Zoe is Anjeli Rapari's fiancée. She acts protectively toward Anjeli when Anjeli is hospitalized with COVID-19, initially refusing to give out her phone number to the investigators seeking information about Adam.