Sungmi and Sunggyu are siblings. Their parents left them at the orphange. And when Sunggyu was old enough to support him and his sister, they moved out. 

Sungmi, 20, is a senior in high school. She studies at a very prestigious school, thanks to her scholarship. She always tops her class, and the whole school in rankings with her intelligence. So Sunggyu almost pays nothing for her studies.

Sunggyu, 24, has band. He is the lead singer of the band Infinite. He made it in the band after he auditioned. And after some weeks, they made him their leader. He met Dongwoo at the orphanage, but he was adopted earlier so they had to part ways. They met again when they accidentally bumped into each other at the bus stop. He asked Sunggyu if he wanted to join their band since their lead vocalist had to leave them for personal reasons. 
He loves his sister very much. He is closest to Myungsoo since he's the youngest in the band, and he always wanted to have a younger brother. He trusts him the most out of all his co-members.

Myungsoo and Sungmi are the best of friends. They met way back in middle school, and they clicked instantly. Both of them are top students. They both entered the same high school. They always mock, tease and fight (in a friendly way..) .. but at the end of the day, they go back to being 'normal'. Myungsoo is a very rich kid, coming from a family who owns big establishments and numerous companies. His family adores the Kim siblings so much they treat them like family, and they have the same family name.

Infinite consists of Sungyeol, 21, their drummer. Woohyun, 21, their pianist. Dongwoo, 23, their bassist and Myungsoo, 20, the band's guitarist, and the visual. 

maybe more characters will come... :)