IU (Lee Jieun):

She's a nice, funny, and cute girl who is poor. She lives with her older brother, Taeyang, who tries to study at school and work. IU has secretly learned how to sing well and play the guitar ever since her parents died. She is in love with one of the most popular student in school: Im Seulong. He has a little sister named Im Yoona, and she's best friends with IU. She also has a close friend named Yoseob, who never tells his true feelings towards her. 

Im Seulong:

He's a smart, popular, and rich guy who has no time for love. He has a little sister named Yoona and is best friends with Dongho. He thinks IU is just those dumb, trouble-making students, and he does not love those kind of students. But one day, he hears singing at the music room, and it's the most beautiful singing he had ever heard. He went to see who it was, but by the time he arrived, the mysterious singer was gone. He tries to find the girl he is searching for, but is fooled by Shin Min Ah, who also loves him. He dates Shin Min Ah, but does not feel love towards her. 

Shin Min Ah:

She's another of those popular girls, and has a crush on Seulong. She is very rich, and sometimes a bit mean. She tricks Seulong by telling him that she learned how to play the guitar and sang at the music room. Soon, Min Ah finds out that it was IU who sang while playing the guitar, and tries to pull Seulong away from her. 

Yang Yoseob:

He's an innocent, childish boy and great friends with IU. He's also good friends with Yoona, too. He isn't poor, nor he is rich. He has loved her ever since they had met, but he is too shy to confess to her. He is also close to Taeyang, who supports him to be IU's boyfriend. But instead of Yoseob helping himself being closer with IU, he helps IU be closer to Seulong. 

Im Yoona:

She's Seulong's younger sister, and she's a brave girl, too. She's rich, just like Seulong, and live in the same mansion as him. She tries to get IU and Seulong together, but realizes that Yoseob has some feelings for IU, too. Yoona already has a boyfriend named Ok Taecyeon. (Taecyeon won't appear much in the story, so he will not be included in the forewords.)

Taeyang:

He is a poor, single man who tries to study at school and work. He works as a singer and a pianist at a lonely bar club. He is an older brother of IU, and remembers an accident when he was young. Their parents died because of the accident, but IU doesn't know about this. He is a good pal with Yoseob, and encourages him to confess his love to IU. He is also very protective, which makes Taeyang forbid IU to be with Seulong and only with Yoseob.