SynopsisCha Dae Woong accidentally releases a gumiho, a legendary fox with nine tails who was trapped inside of a painting by Grandma Samshin, by drawing 9 tails to the fox painting. The gumiho wanted to be human before but was not able to fulfill the request of finding a husband because a rumor was spread claiming that she eats livers of humans. Dae Woong meets a pretty girl after he wakes up from a life threatening fall not knowing that she is the fox he released. When Dae Woong finds out about what he did, he must try to keep her happy and hide the fact that she is a gumiho from everyone. However, because Dae Woong has gumiho's Orb that saved his life, he has no choice but to let her stay with him. As the story goes on, the legendary gumiho (later called Miho) wants to be human. Miho is informed by a veterinarian named Park Dong Joo (who is half human) that she must drink his blood to become a human in 100 days while Dae Woong will have to keep her magical fox orb meanwhile. Moreover, Dae Woong will not be able to date anyone else but gumiho within the 100 days. However, Dong Joo doesn't tell Miho that in 100 days, in order to become human, Dae Woong must die. Trouble brews when Miho and Dae Woong eventually fall in love.
Lee Seung Gi as Cha Dae Woong, the protagonist of the drama, he is a spoiled young man (in his 20s) who lost both of his parents at a very young age and has been with his grandfather and aunt for more than 20 years. He is not interested in going to college; instead, he wants to be an actor. When his grandfather tries to take him to a boarding school, Dae Woong escapes and eventually gets lost. A woman's voice tells him to draw nine tails on a drawing of a fox at a Buddhist temple and in doing so he accidentally releases it. He meets a girl who informs him that she is the fox he released. He finds out that she is a gumiho (later called Mi ho) so he ends up trying to keep her happy (mostly by buying her meat) and hide the fact that she is a gumiho from everyone else. Later, Dae Woong falls in love with Mi ho.
Shin Min Ah as Gu Mi ho (called Mi ho), a legendary nine-tailed fox that has been stuck inside a painting for 500 years. Mi ho is the female protagonist. She dreams of becoming a human and has always wanted to do things humans do. However, when people heard rumors that gumihos eat human livers, no one would go near her. When she meets Dae Woong, she starts to feel like she is human but sometimes exerts her superhuman strength when no one, except for Dae Woong, is around (she was once seen by two children at a park). She becomes Dae Woong's girlfriend and tries to mold their relationship into a normal girlfriend-boyfriend one. Despite the fact that gumihos are ferocious, she is more like the cute and nice type. Since she was made of fire, she is afraid of water.
No Min-woo as Park Dong Ju, a veterinarian at an animal hospital, he is half goblin and half human. He knows a lot of things about human life. Dong Ju is shown to be the antagonist. He tends to speak in a calm way to Mi ho, but doesn't seem to be that way to other people. He tried to capture Mi ho and put her back inside the painting, but when Mi ho tells him that she wants to become a human, he changes his mind. So, he tells Mi ho that she will become human if she drinks his blood and if Dae Woong will keep her marble inside his stomach for 100 days. However, he doesn't tell her that Dae Woong will die if he gives the marble back to her when those 100 days are over. When Dong Ju finds out that Miho is made out of the fire Kil Dal (the girl he loved who made him kill her) left behind, he tries to keep her safe.
Park Soo Jin as Eun Hae In, the female antagonist of the drama, she is Dae Woong's senior. She and Dae Woong had strong feelings for each other for a long time. However, when Dae Woong found out that she was using him, he started to lose interest in her and ignore her. Hae In doesn't like Mi ho, and she becomes infuriated and jealous when Mi ho becomes Dae Woong's girlfriend. She also tried to get Dae Woong to ask her out, but because Dae Woong can't date other people because of Mi ho's marble, she gets rejected a few times. Hae In thinks that she knows Mi ho's difference so she tries to do whatever she can to find out Mi ho's true identity.
Sung Dong Il as Ban Doo Hong, the director of Dae Woong's action school. He is so fond of Dae Woong's skills and gives him special treats. When he sees Mi ho jump high and run fast, he looks for her, and whenever he finds her, she runs away without noticing him. He is eventually seen by Miho, but he doesn't see her. He also has feelings for Dae Woong's aunt, Cha Min Sook. He is also widowed, as he has said that he raised Seon Nyuh by himself.
Hyo Min as Ban Seon Nyuh, Doo Hong's daughter and Dae Woong's best friend. She has feelings for Dae Woong but doesn't seem overtly jealous over the fact that Mi ho is his girlfriend, even though she seems heartbroken at times. She also seems to have become good friends with Miho.
Byeon Hee-bong as Dae Woong's grandfather. He wants Dae Woong to make the right choices in life but accepts his dream of becoming an actor after seeing him mature.Yoon Yoo Sun as Cha Min Sook, Dae Woong's aunt. She does whatever Dae Woong's grandfather tells her to do. She has feelings for Ban Doo Hong but has never been able to tell him. She often ends up making a fool of herself while in the presence of her crush.Kim Ho Chang as Kim Byung Soo, Dae Woong's other best friend. He is supportive of Dae Woong. He has feelings for Seon Nyuh but Seon Nyuh doesn't seem to know.Ahn Jun Ho as Jae Jin, Dae Woong's friend when he was in highschool.