A wealthy, spoiled girl who has a shallow crush on Hae-gi rather than genuine love. She and So-na have known each other since childhood. Sun-mi is aware of So-na's dropout from school and frequently brings it up.
She is somewhat unintelligent, which makes her a source of comic relief in the story. At one point, the other characters tease her about her name, which translates to 'first second third'.
By Volume 12, she and So-na have become close friends, and Sun-mi stops trying to "steal" Hae-gi from So-na. In the end, she becomes very active in dating and gets a new boyfriend every single day.
