Ellie is an observant, puzzle-loving middle schooler living in a small house near San Francisco. Facing changes in her social life as her best friend pulls away, Ellie finds herself thrown into an unusual situation when her grandfather moves in as a teenager. She enjoys cooking, viewing it as a structured, experimental process that mirrors scientific discovery.

Melvin is a brilliant, 76-year-old scientist who has successfully rejuvenated his body to adolescence using a chemical derived from a rare jellyfish. Despite his teenage appearance, he remains a cranky, opinionated man with old-fashioned tastes and a deep passion for scientific discovery. He moves in with his daughter and granddaughter after the police catch him trying to enter his own laboratory.

Lissa is Ellie's mother and a high school drama teacher. She often borrows clothes from the school theatre wardrobe and maintains a busy, somewhat chaotic household. Having spent her life clashing with her father over her choice to pursue the arts instead of science, she finds herself suddenly legally responsible for him when he reverts to adolescence.

Raj is a smart, low-key middle schooler who dresses entirely in black gothic clothing and wears an Egyptian ankh earring. He shares Ellie's school environment and becomes fascinated by Melvin's situation. He eventually agrees to serve as Melvin's laboratory assistant in exchange for participating in their unusual plans.

Brianna is Ellie's childhood best friend who recently cut her hair short and threw herself into middle school volleyball. Enthralled by her new sport and teammates, she abruptly distances herself from Ellie, leaving a sudden gap in Ellie's daily routine.

Ben is Lissa's kind and highly educated boyfriend. He holds a doctorate from MIT but chooses to work at a Silicon Valley startup making video games, a career path that instantly draws Melvin's ire. He remains patient and polite despite the difficult family dynamics.

Ellie's father is a stage actor currently touring in the role of Javert in a national musical production. He supplements his acting income with carpentry skills. Though he and Lissa are divorced, he maintains a warm relationship with Ellie and occasionally helps fix things around Lissa's house.

Nicole is a high school student in Lissa's drama class who works as Ellie's babysitter. She focuses heavily on her own teenage interests and promptly quits her babysitting job upon securing employment at the local mall.

Momo is a middle school student who shares a science lab table with Ellie. The two girls bond over their shared classes, providing Ellie with a quiet, steady social connection.

Mr. Ham is Ellie's middle school science teacher. He volunteers for an organization campaigning against nuclear weapons, teaching Ellie that scientific advancements can sometimes carry destructive consequences.

Ananda is Raj's older brother who has a driver's license and a car. His access to a vehicle makes him a valuable, albeit unwitting, asset to Melvin's schemes.