Charlie is a fifteen-year-old high school freshman who observes life from the sidelines. Following the recent suicide of his middle school friend Michael, he writes anonymous letters to process his emotions. Highly sensitive and intelligent, he prefers reading books and listening to music over actively participating in social events. He struggles to differentiate between being happy and sad, often absorbing the emotions of everyone around him.

Sam is a high school senior with pretty green eyes who quickly becomes Charlie's closest confidant. She is deeply loyal to her friends and actively participates in the local production of The Rocky Horror Picture Show. Though she tells Charlie she is too old for him, she remains highly protective of his well-being and guides him through his early teenage experiences.

Patrick is an extroverted high school senior and Sam's stepbrother. Known for his hilarious personality, he frequently acts as the life of the party and helps draw Charlie out of his shell. He is secretly seeing the school's quarterback, Brad, and deals with the emotional toll of keeping their connection hidden from their peers.

Charlie's older sister is a high school senior who prides herself on her intelligence and feminist ideals. Despite her strong-willed exterior, she finds herself in a complicated relationship with her boyfriend. She gradually forms a closer bond with Charlie as they begin to share and protect each other's secrets.

Charlie's oldest sibling is a college student playing football at Penn State. His room at home is covered in posters of cars and models, reflecting his stereotypically masculine interests. Though he and Charlie do not talk much, his presence is highly valued by the family, and his return home often stops domestic arguments.

Mary Elizabeth is a senior, a practicing Buddhist, and Sam's best friend. She runs a fanzine called Punk Rocky and coordinates the local Rocky Horror Picture Show performances. Highly opinionated and talkative, she often centers conversations around herself and her interests, ranging from underground punk music to foreign films.

Aunt Helen is Charlie's late maternal aunt, who lived with his family during his childhood following a period of personal hardship. She frequently bought Charlie books and allowed him to stay up late to watch television. Her death in a car crash on Charlie's seventh birthday leaves a lasting mark on his psychological state.

Bill is Charlie's advanced English teacher, who recognizes his student's gifted nature early in the school year. He provides Charlie with a steady stream of extracurricular novels and asks him to write essays on them. Acting as a mentor, Bill encourages Charlie to actively participate in life rather than just observing it from the sidelines.

Michael is Charlie's friend from middle school whose sudden suicide occurs just before the start of the novel. Before his death, he and Charlie used to walk around their neighborhood looking into windows and inventing stories about the residents. His absence leaves a significant void in Charlie's life.

Charlie's mother is a caring but often quiet presence in the household. She harbors a deep sadness over the tragic death of her sister, Aunt Helen, and worries frequently about her children's emotional well-being. She tries to maintain peace within the family, especially during stressful holiday gatherings.

Charlie's father is a traditional, emotionally reserved man who occasionally reveals a softer side to his youngest son. He expects his children to be tough and handle their own problems, but he is fiercely protective of his family when outside threats arise.

Brad is the popular quarterback of the high school football team. He maintains a public image of traditional masculinity and popularity, which heavily conflicts with his private life. He strictly separates his public persona from his hidden romantic connections.

Bob is an older friend of Sam and Patrick who frequently hosts parties and supplies drugs to their social circle. He attends the local community college part-time and aspires to become a chef, maintaining very specific personal habits.

Craig is Sam's college-aged boyfriend. He likes to drink red wine and works as a model to pay for his art school education. Charlie heavily scrutinizes Craig, noting that the older boy often seems disinterested in what Sam actually has to say.

Charlie's maternal grandfather lives in a retirement home and regularly attends family functions, where his heavy drinking often leads to uncomfortable or offensive comments. Despite his difficult exterior and abusive past, he holds a deep, quiet pride for his family members.

Susan is a girl from Charlie's middle school days who previously dated his friend Michael. Over the summer before high school, she changes her appearance and begins acting ditzy to attract the attention of popular boys, entirely severing her past friendship with Charlie.