Hayato Kawajiri is an eleven-year-old boy who becomes a pivotal figure in the final arc of the story. He is the son of Kosaku and Shinobu Kawajiri, living in the town of Morioh. His life takes a dark turn when he discovers that his father has been killed and his identity stolen by the serial killer Yoshikage Kira.

Hayato is defined by his quiet and introverted personality. He spends most of his time alone in his bedroom, often wearing headphones and reading manga, and according to his mother, he has no close friends. His relationship with his parents is strained, leading him to secretly observe them using electronic devices he has planted around the house. This habit of surveillance, born from his solitary nature, is ironically what first alerts him to the subtle but disturbing changes in his father's behavior after Kira takes over.

Despite his withdrawn demeanor, Hayato possesses extraordinary bravery and intelligence when faced with a crisis. He is a logical and resourceful thinker, capable of formulating complex plans under immense pressure. His primary motivation is a deep-seated desire to protect his mother, Shinobu, and rid his town of the threat Kira poses, even though his relationship with her is shown to be somewhat distant. This sense of responsibility drives him to take on a killer with supernatural powers without any abilities of his own.

Hayato does not possess a Stand and cannot see them, which makes his contribution to the story even more remarkable. His role shifts from that of a suspicious observer to a central strategist and ally to the protagonists. After Kira gains a new ability called Bites the Dust, Hayato becomes trapped in a time loop. Every time Kira's true identity is revealed to someone through Hayato, time rewinds, resetting the day and killing the person who learned the truth. Hayato experiences this loop repeatedly, watching helplessly as potential allies like the manga artist Rohan Kishibe are killed. In his desperation to break the cycle, he even attempts to take his own life.

His intelligence ultimately becomes the key to Kira's downfall. Using his knowledge of the time loop, Hayato devises a multi-layered plan. He captures a unique plant-based creature named Stray Cat and learns to use its ability to fire invisible air bullets. He also makes a crucial phone call to Josuke Higashikata's house, leaving the receiver off the hook. He then confronts Kira, provoking him into proudly declaring his real name, Yoshikage Kira, out loud. Josuke and his friends hear the confession through the open phone line, arriving in time to attack, forcing Kira to recall his Stand and cancel the time loop.

Once freed from the loop, Hayato transforms from a victim into a key ally. He assists Josuke during the final battle, providing critical tactical advice despite being unable to see the supernatural forces at play. He deduces a limitation of Kira's bomb ability and performs an act of incredible bravery, throwing himself onto a mortally wounded Okuyasu Nijimura, knowing he is a live bomb, to give Josuke a chance to save him. Through these actions, Hayato evolves from a lonely, isolated boy into a courageous and determined hero. Despite all he has been through and his knowledge that his real father is dead, he is last seen tearfully telling his mother that he will wait for his father to come home with her.