You - 16 years old high schooler, who lives alone with your older brother in Seoul as your parents live abroad. You dream to be a psychology doctor one day. You secretly have a crush on your brother's best friend Park Seo Joon. Will he return your feelings?

Na In Woo - As your older brother, who is 3 years older than you and majoring in business. He has been Park Seo Joon’s best friend since they were kids. He is a caring, funny, and soft person. 

Park Seo Joon - Your secret crush since you were 15. He is an orphan, who was adopted by a doctor couple when he was 5. Since then, he dreams about becoming a pediatrician and majoring as a pre-med student at the same university as In Woo. He seems cold-hearted but at the same time very warm. 

Lee Kyung Mi - Seo Joon's first love.