Jackie is a driven, organized teenager from New York City who loses her parents and older sister in a sudden car accident. She moves to a rural Colorado ranch to live with the Walter family and their twelve children. A former boarding school student, she relies on structure and meticulous planning to suppress her overwhelming grief. She struggles to adapt to the unpredictable household while sorting out her conflicted feelings for two of the brothers.

Cole is a confident, attractive older Walter brother who works as a mechanic and restores a vintage car in the family garage. He formerly held a football scholarship before a severe leg injury ended his athletic career. Following the injury, he adopts a carefree, womanizing attitude to mask his feelings of vulnerability. He frequently challenges Jackie's rigid boundaries and encourages her to take risks.

Alex is a quiet, dependable Walter brother who shares an anatomy class and a love of books with Jackie. He plays video games and competes on the baseball team. He harbors an ongoing rivalry with his older brother Cole. He provides Jackie with a sense of safety and comfort in the loud house, though his lingering anger over a past relationship clouds his judgment.

Nathan is the fourteen-year-old middle Walter brother and an early ally to Jackie. He runs with her in the mornings and explains his brothers' confusing behaviors, helping her adjust to ranch life. He manages an epilepsy diagnosis and offers a gentle, neutral presence in the busy household.

Katherine is Jackie's mother's childhood friend who takes Jackie in following the fatal car accident. She lives on a sprawling Colorado ranch and manages a loud, active household of boys. She works as an artist, painting vibrant murals across the house and barn to make the space feel warm and welcoming. She sacrifices her art studio so Jackie can have a private bedroom.

George is Katherine's husband and the patriarch of the Walter family. He helps manage the sprawling property and the twelve children living under his roof. He offers Jackie a friendly welcome and tries to maintain peace when conflicts erupt between the older boys.

Danny is Cole's twin brother. He is an introverted teenager who enjoys reading Shakespeare and acting in the school's theater productions. He earns the lead role of Romeo in the spring play and dreams of attending a summer theater program in New York City.

Will is the eldest Walter sibling who no longer lives on the family ranch. He resides in a nearby apartment with his fiancée, Haley. He acts as a mature, welcoming older brother figure for Jackie and offers his apartment as a refuge when she needs space from the household chaos.

Isaac is a biological cousin to the Walter boys who lives on the ranch. His mother abandoned him and his brother, and his army officer father left them in Katherine and George's care. He participates in household pranks, including picking the bathroom lock to steal Jackie's clothes and towel.

Lee is Isaac's brother and a biological cousin taken in by Katherine and George. He initially directs hostility toward Jackie, openly complaining about her presence and the fact that Katherine gave up her art studio for her. He struggles with feelings of abandonment regarding his own parents.

Jack is a younger Walter brother and a twin to Jordan. He carries a video camera and frequently tries to film the daily chaos of the household, including an attempt to catch Jackie running from the bathroom.

Jordan is Jack's twin brother. He contributes to the wild atmosphere of the house by keeping a pet snake, which he accidentally lets loose during Jackie's first family dinner.

Parker is the only female child in the Walter family. She maintains a tomboyish appearance and frequently plays video games with her younger brothers, bonding with Jackie over rounds of Mario Kart.

Zack is one of the younger Walter boys. He plays soccer and hide-and-seek with his siblings around the large ranch house.

Benny is a young Walter sibling. He hides in the linen closet with Jackie during a game of hide-and-seek to evade his brother Zack.

Mary is a high school student who recently broke up with Alex to date Cole. She acts with hostility toward Jackie, making cruel comments about Jackie's deceased family to warn her away from Alex.

Riley is a high school student who befriends Jackie in art class. She offers Jackie advice about embracing the unpredictable nature of life and love, acting as a sounding board for Jackie's anxieties.

Heather is a student in Jackie's art class who helps initiate their friendship. She suggests having a sleepover at the Walter house to satisfy her curiosity about the large family.

Sammy is Jackie's best friend and former roommate from her all-girls private school in New York City. She stays in touch with Jackie over the phone, offering advice and encouraging her to develop crushes as a way to heal.

Lucy is Jackie's older sister who perished in the car accident alongside their parents. Jackie viewed her as flawless and constantly pressured herself to achieve perfection in order to earn the same parental affection Lucy received.

Haley is Will Walter's fiancée. She lives in an apartment with him and helps look after the younger siblings during the family's busy weekends.

Olivia is a local high school student who maintains a casual romantic arrangement with Cole Walter.

Erin is a girl who visits the Walter ranch to ask Cole if they are still hanging out, demonstrating his habit of dating multiple girls casually.

Kim is a high school student and friend of Alex Walter. She joins Riley, Heather, and Jackie's lunch table and attends the local parties.

Nick is a friend of Cole Walter. He provides rides to school and discusses plans to hang out at the abandoned warehouse.