Male. After Go Ju-tae's death, he grew distant from his close friend Park Seol-ah and lived an ordinary life as a teacher. He eventually wished to "go back to the past," which led to his actual return. He was a perfect model student in his school days, but because of that, he failed to be there for Ju-tae when he was struggling and lonely. He is one year older than Ju-tae.

Female. A student who returned to the past with Lee Seo-baek. Before returning, she worked as a police officer. She is quite skilled with her hands, especially when it comes to packing lunch boxes. She is one year older than Ju-tae.

Male. The student who serves as the catalyst for the loop. He is a friend of Lee Seo-baek and Park Seol-ah, and is one year younger than them. Unlike the other two, he has looped hundreds of times, but every time he returns to the past, he inevitably dies on December 31st—the day he first took his own life. When Ju-tae dies, everyone repeating the loop is sent back to the day of the entrance ceremony. He suffers from school violence and appears to have a difficult family life. He is the same age as Eun-hyun.

Male. A student who, like Go Ju-tae, has been through the loop hundreds of times. He has known Ju-tae since elementary school. He has a strong delinquent vibe and gets irritated easily. Although he snaps at Ju-tae often, he secretly takes good care of him. He is the same age as Ju-tae.