Eric is a high school swimmer who used to be overweight, earning him the nickname "Moby" from his peers. He is deeply introspective and loyal, struggling to balance his newfound social acceptance on the swim team with his enduring bond to his hospitalized childhood friend. He often wrestles with moral and ethical dilemmas in his Contemporary American Thought class, learning to question his own assumptions.

Sarah is Eric's fierce, resilient childhood friend who suffered severe facial and hand burns as a toddler. Once the steely co-editor of an underground middle school newspaper with Eric, she has suddenly gone silent and been hospitalized. Her current state prompts Eric to seek answers about her well-being and home life.

Virgil is Sarah's cruel and intimidating father. He has a dark presence and a history of volatile behavior, commanding fear from those around him. His oppressive nature drives much of the anxiety surrounding Sarah's sudden withdrawal from school and subsequent hospitalization.

Mark is a rigid, evangelical student on the swim team and in the Contemporary American Thought class. He strictly adheres to dogmatic religious views, frequently using them to judge his peers. His need to project an image of absolute perfection masks deep internal pressures and fears of failure.

Ms. Lemry is Eric's dedicated swim coach and teacher. She cultivates a classroom environment that encourages teenagers to re-examine their beliefs and push past their assumed limits. Honest and non-judgmental, she is the primary adult Eric trusts with his deepest concerns regarding his friends.

Steve is Eric's current best friend and a fellow high school swimmer. The son of a pastor, Steve has a quietly reflective but irreverent faith, formed in the wake of his older brother's sudden passing. He uses his sharp wit and provocative bumper stickers to challenge dogmatic classmates who view the world in absolute terms.

Eric's mother is a warm, approachable parent who raises Eric single-handedly after his father left. She maintains an open dialogue with her son, offering him a safe space to discuss his worries about his friends and his complex social life.

Mr. Ellerby is Steve's father and a local pastor. Unlike other religious figures in the community, he possesses a tolerant, flexible outlook on faith. He strongly advocates for the separation of church and state, pushing back against school administrators who try to force religious conformity.

Mr. Brittian is Mark's father, a man who shares and enforces his son's narrow, inflexible interpretations of faith. He places immense pressure on Mark to uphold strict moral standards and quickly searches for scapegoats when problems arise, rather than examining his own parenting.

Jody is a high school student caught in a moral and emotional quandary. Initially appearing completely in sync with her boyfriend Mark's strict worldview, she secretly harbors immense personal guilt. She begins to question the judgment she faces from Mark and finds herself drawn to Eric's open-mindedness.

Dale is a former bully who failed eighth grade multiple times before eventually dropping out. Initially a tormentor who mocked Eric's weight and Sarah's scars, he formed an unlikely alliance with them against the school administration. He now serves as a trusted, rough-around-the-edges informant for Eric.

Mr. Mautz is the deeply resentful vice principal of the high school. He views Eric as a chronic troublemaker and heavily favors devout, rule-abiding students like Mark Brittian. He frequently attempts to silence classroom discussions that challenge his conservative worldview.

Carter is the new boyfriend of Eric's mother. Initially viewed with typical teenage skepticism by Eric, Carter gradually reveals unexpected depth, offering Eric valuable support and understanding drawn from his own painful childhood experiences.