Hiromi Sugita is a classmate and close friend of the protagonist, Satoru Fujinuma, in the elementary school timeline of the story. He is one of the key children at the center of the serial kidnapping and murder cases that Satoru attempts to prevent. In the original, unaltered history, Hiromi is known as the third victim of these crimes. His fate becomes one of the critical events Satoru is determined to change.

Physically, Hiromi has a noticeably androgynous or feminine appearance, with delicate features, black hair cut to his ears, and blue eyes. This appearance is a significant aspect of his character, as it initially leads investigators in the original timeline to believe the killer mistook him for a girl. This misconception becomes a deliberate part of the true culprit's strategy to mislead the police. His personality is generally optimistic, cheerful, and friendly, though he can also be somewhat shy. He is an intelligent student with a particular fondness for reading. One of his notable traits is his skill at board games, including chess and Othello, where he proves to be a formidable opponent.

Hiromi's motivations are rooted in friendship and a personal understanding of loneliness. Because his parents are often busy, he spends a great deal of time alone and therefore deeply understands what it feels like to be isolated. When Satoru, after using his Revival ability, enlists his friends' help to protect the endangered Kayo Hinazuki, Hiromi willingly and eagerly cooperates. He expresses happiness at being able to help, and he quickly believes Satoru's warnings about a potential kidnapper, becoming a reliable and active source of support.

Within the story, Hiromi serves as one of the original victims that Satoru is trying to save, which places him at the heart of the central mystery. He is a core member of Satoru's group of friends, which includes Kenya and Osamu. His role evolves from a passive victim into an active participant in changing the future. By believing in Satoru and helping to protect Kayo, the circumstances that led to his own death are erased. His key relationships are primarily with his friends in the group; his bond with Satoru is particularly important, as Satoru's actions directly alter his fate.

Hiromi undergoes a significant transformation in the altered timeline. Having avoided his original fate, he is able to grow up. Years later, as an adult, he becomes a medical doctor, a career path he chose specifically because of Satoru, who ended up in a coma after preventing the initial tragedies. In a surprising development, the adult Hiromi marries Kayo Hinazuki, the primary girl Satoru sought to protect, and they have a child together named Mirai. This outcome demonstrates how Satoru's intervention created a new, happier future for both of them. As for abilities, aside from his academic intelligence, Hiromi is notably skilled at strategic board games like chess and Othello, which highlights his methodical and thoughtful nature.