"Wanna hang out?"

"Sorry...I have a date with Minwoo hyung..."

"I saw you talking to that girl..."

"Hyung, I swear...I just asked about the assignment..."

"You are really old..."

"You are really young..."

"Minwoo is mine!"

"No, Minwoo is mine!"

Lee Sunho/Andy

Student

Brother of Lee Minwoo

Best friends: Kim Dongwan and Kangta

Park Chungjae/Junjin

Popular Student

Boyfriend of Lee Minwoo

Friend: Andy

Best friend: Hyesung

Lee Minwoo

Popular Student

Andy's hyung

Junjin's boyfriend (the jealous and possessive type)

Friend: everyone who likes Junjin

Kim Dongwan

Smart student

Andy's other best friend

Seductive 

Has a huge crush on Lee Minwoo

Rival: Other best friend Kangta

Jung Pilkyo/Hyesung

Emo/popular kid

Very rich

Junjin's best friend

Arranged to marry some old dude

Old Dude AKA Mun Junghyuk AKA Eric

heir of his father's company

"slightly" older than a young guy he is going to marry

Best friend to Lee Jihoon

Ahn Chilhyun/Kangta

Dumb Student

Best friends: Andy and Dongwan

Rival: Dongwan

Crush: Lee Minwoo (and teacher Lee Jihoon not so secretly)

Seductive 

Charming music teacher Lee Jihoon

Eric's best friend

Teaches all of the above, excluding Eric