A rich, spoiled girl who likes Hae-gi but it is more of a shallow crush than true love. She and So-na knew each other from when they were young. Sun-mi knows about So-na's dropping out of school and never fails to mention it. She is rather dim-witted, and so is used for comic relief. At one point, the other characters make fun of her name because it means 'first second third'.
By Volume 12, she and So-na have become really close friends, and Sun-mi stops her attempts on "stealing" Hae-gi from So-na. In the end, she is dating like crazy and has gets a new boyfriend every single day.