About the characters when they were college students. 

Park Jiyeon is a girl that wanted to save lives. Her parents had died when she was 5. When she heard about her parents death she was sad. When she turned 15 she decided to become a surgeon. She wanted to save lives. She wanted to becomes surgeon because she doesn't want anyone to feel the depression that she felt. 

Krystal is a pretty girl that loves medicine. She would always study what types of medicine there were. Her mom was a pharmacist and her mother would always say these were potions that helped people. So Krystal wanted to give those potions to the people that were sick. That is why she wanted to becomes pharmacist. 

Then their is a girl named Naeun. She does not go to the same college as Hongbin, Jiyeon, or Krystal. She goes to the same college as Taemin and Kai. Naeun doesn't want to be a surgeon or a pharmacist, she wants to be a nurse. Her father is a surgeon, but her mother is a nurse. She really wanted to follow her mother's steps.  Nevertheless, really loves her mother and she wanted to work with her mother. 

Taemin and Kai were twin brothers. However, they were nothing a like. They were the opposite of each other. Taemin was a caring and nice person. However, Kai was an arrogant, spoiled, and not so caring person. Kai hates Taemin for many reasons. The main reason is; his father always compared him with Taemin. Kai really hates that. He really hates it. So Kai always tries to beat Taemin. However, he could never beat Taemin. 

Lee Hongbin wanted to become a surgeon, because he loved to learn about the body parts and what they do. He, nevertheless, loved the concept of surgery. His neighbor was a surgeon and his neighbor would always tell Hongbin the miracles of surgery. Hongbin loved the stories, so he wanted to become a surgeon. 
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The Present Characters

Park Jiyeon studied overboard for nine years. She left her best friend Krystal and the guy that she liked. She decided to leave Belgium and come back to her country. She is a cardiothoracic surgeon.  

Krystal is a pharmacist and she is Jiyeon's best friend. She is the only person that contacted Jiyeon when she was in Belgium. Krystal is really nice towards everyone. You can not hate Krystal because she is bright and bubbly. 

Naeun is a cardiac care nurse. She really is the girlfriend of Hongbin. She belongs to a rich family, her father is the highest or best surgeon of this hospital. She is nice, rich, smart, and pretty. She is the perfect woman.  

Taemin is a cardiothoracic surgeon. His father is the lawyer of the hospital Taemin works at. He has a twin brother that is also a surgeon. Taemin is nice and really funny. 

Kai is the twin brother if Taemin. Kai is a general surgeon. He does not like Hongbin and Taemin. He thinks if them as his rivals. He is arrogant and sometimes very bossy 

Hongbin is a cold hearted cardiothoracic surgeon. He works with Taemin. Hongbin is not a social person. He only talks when there are meetings. He doesn't really talk to his surgeon team or anyone in this building, except for Naeun. Naeun is his girlfriend.  He reveals his real personality only one person and that is Naeun