Male Age: 12 Original Name: Ryu Han-gyeol A genius doctor who dreamed of being the best. Trapped in a body that suddenly became younger, he now lives as an elementary school student. His body has shrunk, but his mind is still filled with countless diagnoses and medical options. Thanks to his handsome looks, even adults he meets for the first time feel compelled to strike up a conversation. He has a scar on his forehead and is currently hiding the fact that he turned from an adult into an elementary student.

Gender: Male Age: 12 He hates losing to anyone when it comes to studying. He tries to act nonchalant in front of Han-gyeol, who appeared out of nowhere, but his whole body radiates a sense of crisis that he might lose his top spot. He is lactose intolerant. He has a gentle and timid-looking face, but he shows a surprisingly haughty and exemplary side.

She looks like a boy but is a girl. Age: 12 Gender: Female She jumps into new things without hesitation and can't hold back her curiosity. She clings to Han-gyeol and talks to him without reserve, and she is the type to say whatever is on her mind even in front of adults. Due to her androgynous appearance and short haircut, she is sometimes mistaken for a boy.

Male. A passionate male teacher who left the city to come down to a rural school with few students. He is always busy taking care of even the smallest things himself, vowing not to do his classes half-heartedly.

Male. A professor and the mentor who has watched over Han-gyeol longer than anyone else. He enjoys finding the principles of how the body works even in daily life. He is flustered when he sees his disciple returned to a younger form, but he soon accepts it and becomes his most reliable support.