Seon Yunjae is a teenager living in Suyu-dong who was born with a neurological condition called alexithymia. His underdeveloped amygdala prevents him from identifying, understanding, or fully experiencing emotions like fear or anger. He lives above a used bookstore with his family, where he attempts to learn the rules of normal human interaction through memorization and rote practice. He observes the highly emotional world around him with a literal, detached, and highly logical perspective.

Gon is a newly enrolled transfer student who immediately establishes a hostile reputation at school. He carries intense psychological trauma from years spent separated from his family and passed through various unstable living situations. He uses profanity, aggressive posturing, and threats to keep his peers at a distance and protect himself from further emotional pain. Despite his tough exterior, he feels emotions intensely and desperately craves genuine connection.

Mom is a protective, highly anxious woman who operates a used bookstore to support her family. Following the death of her husband, she dedicates her energy to ensuring her son can survive in a society that rejects differences. She writes out detailed scripts, creates flashcards, and papers their house with instructions to teach him how to fake normal emotional responses. Her intense push for normalcy stems from a deep fear for his future safety.

Granny is a large, loud, and physically imposing woman who moves into the bookstore to help raise her grandson. She brings a chaotic, festive energy to the household, enthusiastically decorating their living space for every possible holiday. She affectionately refers to her grandson as a monster, ignoring conventional politeness in favor of blunt honesty. She provides a sturdy, grounded presence that balances her daughter's high-strung anxiety.

Dora is a high school student whose entire life revolves around her passion for running. She serves as the sole member of the school's unfunded track team and completely ignores the social hierarchy of her classmates. She frequently wears glasses that she uses as a physical barrier to block out the rest of the world. She maintains her athletic pursuits even though her parents actively discourage her interests.

Dr. Shim is a former heart surgeon who operates a popular bakery located directly above the family's used bookstore. He abandoned his medical career following the sudden death of his wife, choosing instead to bake as a way to process his grief. He maintains a quiet, optimistic demeanor and keeps unusual hours at his business. He acts as a patient, non-judgmental adult figure for the people in his building.

Professor Yun is an older academic whose life was derailed by the disappearance of his toddler many years ago. He desperately attempts to manage his dying wife's grief while struggling to accept the reality of the son who eventually returned to him. He cares deeply about social respectability, which causes severe friction within his family. He often resorts to harsh discipline when his expectations are not met.

Mrs. Yun is a formerly successful journalist whose physical and mental health deteriorated completely after she lost her young child in a park. She spends her days confined to a hospital room, consumed entirely by the memory of her missing son. Her fragile condition leads her husband to construct elaborate comforts to bring her peace.

Steel Wire is an infamous, violent criminal who operates out of the city's rougher districts. He maintains a dangerous reputation that attracts angry, marginalized teenagers who wish to emulate his lack of fear. He hides a surprisingly beautiful face behind a mask and commands obedience through intimidation and pain. He views the young people who flock to him as entirely disposable entertainment.

Steamed Bun is a street-smart teenager who walks the dangerous, unstructured neighborhoods outside the school's purview. He keeps track of the local youth and knows how to locate people who have dropped off the map. He understands the severe risks involved in associating with the area's older criminals.