Theodore is a 17-year-old high school student who wrestles with untreated mental illness and suicidal impulses. Nicknamed "Freak" by some peers, he frequently adopts different personas and throws himself into intense bursts of manic energy to avoid falling into depressive periods he refers to as being "Asleep." Despite his internal struggles, he is highly intelligent, observant, and deeply compassionate toward Violet.

Violet is a former cheerleader and popular student who feels entirely disconnected from her old life following the death of her older sister in an auto accident. Burdened by survivor's guilt, she abandons her passion for writing, withdraws from her friends, and refuses to ride in cars. She wears her late sister's oversized glasses as a form of penance until Finch challenges her to reengage with the world.

Charlie is a bright, athletically talented student who rejects sports teams to avoid conforming to racial stereotypes, opting instead for chess and the yearbook club. He is a serene, even-tempered presence in Finch's life who offers steady loyalty and low-pressure support without using therapeutic jargon.

Brenda is an unapologetic nonconformist who dyes her hair bright colors and wears a nose ring. She stands in sharp contrast to the school's popular crowd. She fiercely protects Finch and offers blunt, honest advice to those in his circle.

Amanda is a popular cheerleader whose social status relies on her assertive exterior and her family's wealth from a chain of liquor stores. She hosts large drinking parties and runs with the school's elite crowd, though she hides deep insecurities, an eating disorder, and personal struggles behind her mean-girl facade.

Linda is the 41-year-old single mother of Theodore, Kate, and Decca. Emotionally devastated by her recent divorce from her abusive ex-husband, she works two part-time jobs and copes by relying on wine and phone conversations. She is loving but overwhelmed, often missing the warning signs of her son's mental illness.

Ted is a former professional hockey player who recently left his family to marry a much younger woman. He is quick to anger and has a history of physical and emotional abuse toward his ex-wife and children. He maintains a tense, hostile relationship with Theodore, who actively tries to provoke him.

Kate is Finch's 18-year-old sister who effectively functions as the primary caretaker of the Finch household. She manages the family's meals and often covers for her brother's depressive episodes, attempting to maintain stability in a fractured home environment.

Gabe, frequently called "Roamer," is a popular athlete and Finch's sworn enemy since middle school. He frequently instigates physical altercations and verbally harasses Finch, using his status on the baseball team to avoid disciplinary consequences.

Ryan is Violet's handsome, popular former boyfriend. He struggles to accept the end of their relationship and frequently attempts to reignite their romance, representing the conventional, superficial high school life Violet is trying to leave behind.

Eleanor is Violet's late older sister, whose recent death in a car crash severely traumatized Violet. Before the accident, the two sisters were best friends who co-founded an online magazine and shared similar academic and creative ambitions.

Decca is Finch's eight-year-old sister who receives very little parental supervision. She copes with the tension in her family by collecting books and physically cutting out all the negative or mean words so they do not mix with the good ones.

Mr. Embry (nicknamed "Mr. Embryo" by Finch) is the high school counselor assigned to Theodore Finch. He makes well-intentioned but often inadequate attempts to monitor Finch's mental state, struggling to penetrate the teenager's elaborate defenses and disguised symptoms.

Mrs. Kresney is Violet's consistently smiling school counselor. She focuses heavily on Violet's academic progress and extracurricular reengagement, utilizing a generic approach that fails to address the root of the teenager's profound grief.

Mr. Markey is Violet's father, who struggles to process his own grief while attempting to guide his surviving daughter. He places heavy emphasis on academic achievement and rule-following, eventually becoming protective when he realizes the extent of Finch's instability.

Mrs. Markey is Violet's mother, a writer who actively pushes her daughter to resume a normal social life. She attempts to reconnect with Violet by helping her brainstorm ideas for a new online magazine, hoping to spark her daughter's lost creativity.

Mr. Black is a high school geography teacher who forces Violet and Finch to partner on an oral presentation. By refusing to excuse Violet from the assignment due to her extenuating circumstances, he inadvertently sparks their state-wide wandering project.