Kenya Kobayashi is a primary school classmate and close friend of Satoru Fujinuma. He is a thoughtful, quick-witted, and highly observant child, possessing a level of brilliance and emotional intelligence that stands out even among his peers. Physically, he is depicted as a boy with blond or yellow hair that reaches his ears and brown eyes, often wearing earmuffs or a coat.

Kenya comes from a family with a lawyer father, a homemaker mother, and a younger sister, and he holds a deep respect for his hardworking father. His background is marked by a significant personal event: his father once defended a man in a murder case, pleading not guilty by arguing against the suspect's character, a claim no one believed. This experience deeply influenced Kenya, fostering a strong sense of justice and a childhood dream of becoming a lawyer to prevent such miscarriages of justice from happening again. Despite his high performance in both academics and sports, Kenya often felt a sense of emptiness from the praise he received, which led him to look down on others from a distance and believe himself to be more grown-up than he was.

In his youth, Kenya is a calm and level-headed individual who usually wears a neutral expression. His sharp perception allows him to quickly notice details others miss, such as the bruises on their classmate Kayo Hinazuki indicating her abuse at home. However, despite being one of the first to discover this, he initially kept his distance and took no action, believing in cautious observation. This changes when he sees Satoru actively and courageously intervening in Kayo's life. He recognizes that Satoru's interest is not born of simple childhood affection but a determined will to protect, which inspires him to abandon his passive role.

Kenya becomes one of Satoru's most invaluable allies, serving as the reserve or backup that Satoru can rely on. While Satoru is driven and prone to impulsive actions, Kenya provides calm, rational advice and strategic thinking, often preventing his friend from taking reckless steps. He is the first to notice the drastic change in Satoru's personality after the time leap, sensing that his friend has become much more mature and rational, even testing him on his knowledge to confirm the shift. Despite his suspicions, Kenya chooses to trust Satoru and offers his full cooperation, providing crucial support in the plan to save Kayo and uncover the true culprit behind the serial kidnapping cases. It is Kenya who suggests the abandoned bus as a hideout for Kayo and who later provides a critical alibi for Jun Shiratori, testifying that he saw him on the day of a crime, which helps exonerate an innocent man.

After Satoru falls into a fifteen-year coma following his confrontation with the real culprit, Kenya is deeply affected. True to his childhood ambition, he becomes a lawyer and dedicates himself to pursuing the same serial killer, working alongside a man named Sawada. When Satoru finally awakens with memory loss, an adult Kenya is there, displaying his enduring kindness and support. He helps Satoru piece together the past, carefully avoiding feeding him false information that could corrupt his memories of the true antagonist. Kenya's key abilities are his exceptional intelligence, observation, and strategic thinking. He is one of the only people Satoru confides in about his Revival ability, and he is noted as the only member of their friend group to correctly identify the true culprit. His character develops from a brilliant but detached observer into a courageous and active protagonist who learns from Satoru's example that taking action is more valuable than just watching from afar.