Aristotle is a fifteen-year-old Mexican American boy living in El Paso, Texas. He often feels isolated from his peers and describes himself as a mystery to everyone, including himself. He spends his time brooding, writing in his journal, and wondering about his family's unspoken history. Beneath a tough and quiet exterior, he cares deeply about others but struggles to articulate his feelings.

Dante is an articulate and highly emotional teenager who reads poetry and readily expresses his feelings. Unlike Aristotle, he has an open relationship with his parents but feels insecure about his Mexican heritage. He is highly empathetic, going so far as to cry over a dead bird, and views the world with a mixture of idealism and artistic sensitivity.

Mr. Mendoza is Aristotle's father, a quiet mail carrier who served as a Marine in the Vietnam War. He keeps his emotions tightly guarded and rarely speaks about his past or the trauma he carries. Despite his emotional distance, he loves his son and occasionally bonds with him over shared moments of silence.

Mrs. Mendoza is a high school teacher who values education and maintains a strict but practical household. She shares a similar sense of humor with Aristotle and worries constantly about his social isolation. She tries to protect her family by avoiding painful topics from their past.

Often called Sam, Mr. Quintana is an English professor and Dante's father. He has a cheerful, affectionate demeanor and openly sings with his son in the car. He provides a warm environment that contrasts sharply with the quiet tension in Aristotle's home.

Mrs. Quintana works as a psychologist. She presents a slightly more serious and strict presence than her husband but is deeply observant of those around her. She cares intensely for Dante and extends a protective attitude toward Aristotle as the boys become close.

Bernardo is Aristotle's brother, who is eleven years older and currently serving time in prison. The family enforces a strict code of silence regarding his existence and his actions. His absence leaves a heavy emotional void in Aristotle's life.

Gina is a smart, talkative high school student who has been friends with Aristotle since the first grade. She is nosy about his personal life and uses lighthearted teasing to draw him out of his shell.

Susie is a high school student and close friend of Gina Navarro. She spends time with Gina and Aristotle, frequently participating in their casual banter and social outings.

Ileana is an older girl at Aristotle's high school whom he finds highly attractive. She has a mysterious aura that catches Aristotle's attention as he begins exploring romance.

Ophelia is a relative living independently in Tucson. She holds a quiet significance in the family's history, having provided support to family members during difficult times in the past.

Charlie is a former friend of Aristotle's who has started making poor life choices. He drops by uninvited to offer Aristotle drugs, representing a path Aristotle actively chooses to avoid.

Dr. Charles is the surgeon responsible for treating Aristotle's injuries. He advises patience during the recovery process and remarks on Aristotle's bravery.

Mr. Blocker is a high school English teacher who uses icebreaker questions to learn about his students, unintentionally putting Aristotle on the spot about his summer.

Legs is a stray dog that Aristotle adopts and names in honor of regaining his own mobility. The dog provides Aristotle with silent companionship and comfort during his times of isolation.