Brooke is an ambitious high school senior at the prestigious Waldorf Preparatory School. As the only child of a wealthy, status-oriented family, she feels immense pressure to maintain an image of perfection and continue the family legacy at Yale University. Haunted by a tragic boating incident at her family's lake house six months prior, she struggles to hide the escalating stalking and vandalism she receives while trying to keep her carefully crafted public persona intact.

Claire is Brooke's former friend who transferred to a public school after her family suffered a public disgrace and loss of wealth. In the "Before" timeline, she crashes Brooke's party with a desire to cause trouble and humiliate the Goodwin family. Her tragic drowning in the lake that night serves as the inciting incident that haunts the rest of the characters.

Jena is Brooke's enthusiastic and sociable best friend. Coming from a less prestigious background than the Goodwins, Jena feels freer to pursue her genuine interests in culinary arts rather than chasing Ivy League status. She acts as Brooke's closest confidante and defends her against Claire's antagonism during the summer party.

Dylan is a popular student, soccer player, and early admit to Brown University. He works at his family's bookstore and tends to avoid messy social conflicts. He frequently shows a soft spot for Brooke, giving her his hoodie when she feels overwhelmed at social gatherings.

Felix is Jena's boyfriend and a close friend of Dylan. He is part of Brooke's inner social circle at Waldorf Prep. During the flashbacks to the lake house party, his relationship with Jena experiences severe tension due to past romantic drama, making him a prime target for Claire's manipulative party games.

Brooke's father is a severe, highly ambitious senior lawyer who is actively campaigning for an appointment as a circuit judge. He places immense value on the Goodwin family name and expects absolute perfection from his daughter. He views achievements merely as necessary benchmarks for maintaining their elite status rather than milestones to be celebrated.

Brooke's mother is the principal of the prestigious Waldorf Preparatory School and a Yale legacy alumna. She meticulously curates Brooke's extracurricular activities, such as a local food drive, to ensure her daughter presents the perfect image for college admissions.

Brandon is Claire's grieving brother. Visibly agitated and angry in the months following his sister's drowning, he publicly accuses Brooke of lying about the events of that night. Brooke initially suspects him of being the anonymous stalker making threatening phone calls to her.

Beau is a classmate of Brooke and Jena who hosts the Ivy Day celebration to mark college admissions. He previously shared a brief romantic moment with Jena during a pause in her relationship with Felix, an event Claire uses to spark a fistfight at the summer lake house party.