Kang Min-yeon, the younger sister of a young doctor meets and falls in love with Park Ji-hyuk, whom she met online. However, their love story is bound by the people around them, including their families, who have a hard time accepting. Can they overcome all odds to be together?  

IU as Kang Min Yeon 

She is the younger sister of Kang Mo-yeon, a cardiothoracic specialist who works at Haesung Hospital. She is the close friend of Shin Tae-yong.  She is sweet and very likeable. However, she is also very sensitive when it comes to Ji-hyuk. She is quite oblivious to the erted, inappropriate and hurtful jokes towards her and Ji-hyuk and she loves animals. Her passion is to become a veterinarian. 

Wooyoung as Park Ji Hyuk

He is extremely caring and protective which is one of his many charms. He is pretty much friends with everyone, or everyone considers him as a friend. He, like others, can only tolerate so much. He snaps and is very harsh when he does. He won’t deny but he highly believes in soul mates. His past was something he never wanted to talk about as it only brings him unwanted memories.

Kim So-eun as Im Ki-jung

She is Ji-hyuk’s ex girlfriend. She would do anything to get Ji-hyuk back, even if it means using her son. Once a upon a time, she and Ji-hyuk used to date until something happened between her and Ji-hyuk's best friend, Kwon Shi-kyung.

Song Jae-rim as Kwon Shi-kyung 

He used to be Ji-hyuk's best friend but betrayed him after being caught in an affair with Ji-hyuk's ex-girlfriend, Ki-jung. Their love bear fruit to their son, Kwon Doo-hyun. However, he left Ki-jung and their son for another girl. 

Lee Joon-gi as Shin Tae-yong

A pre-dental sophomore student in Woosang University and is Min-yeon’s friend. He is an introvert and prefers to be always alone. He is often seen in the library alone or with a few of his friends. He has a girlfriend named Kyung-hee.