I will start by describing the characters...

Eunhyuk

He's an average man, with a normal working life in an office. He is drawn to his new colleague who has an unusual, cold personality. He grew up with Hyoyeon, Onew and Doojoon. 

Hyoyeon _{(not your regular hyoraengi here...)}

She works as an artist and relies on Eunhyuk so much that she doesn't think she can live without him. However she does not think that he likes her and is too shy to make a move anyway. 

Taeyeon

She is Eunhyuk's new colleague. She is aware of Eunhyuk's feelings towards her but is not ready for a new relationship because of the recent loss of her boyfriend. She has a tendency to use Onew as a replacement, to fill up the gap that her ex left behind.

Onew

He's Hyoyeon’s protective older brother who loves her more than anything. He has an astonishingly similar character and personality with Taeyeon’s late boyfriend. He really loves with Taeyeon and is willing to be her substitute.

 Doojoon

He is Hyoyeon, Onew and Eunhyuk's childhood friend. He aspires to be an elite agent in their district’s top investigating firm. He has a one-sided love for Hyoyeon, but knows that she likes Eunhyuk so he vows to take care of her like a sister and not let anyone harm her.

Junho

He is a former gang member of the most notorious gang in the district. He was a valued member in his previous gang as he has helped them to avoid the police on many occasions. He really likes Hyoyeon and has quit the group for Hyoyeon. He's also trying to turn over a new leaf.