After their nobleman father is unfairly accused of being a traitor and killed, Yoon Seo-hwa (Lee Yeon-hee), her younger brother Jung-yoon (Lee David) and their maid Dam (Kim Bo-mi) are sent to a gisaeng house. Seo-hwa's first "patron" is Jo Gwan-woong (Lee Sung-jae), the man who betrayed and killed her father. Before Jo Gwan-woong arrives, Dam swaps clothes with Seo-hwa so that she can run away, chased by Gwan-woong's men. Gu Wol-ryung (Choi Jin-hyuk), a mystical forest protector and gumiho, finds Seo-hwa unconscious and, as he had fallen in love with her, protects her. When Seo-hwa wakes up, she too falls in love with him and marries him after he tells her that both Dam and Jung-yoon were able to run away and are safe. Actually, Jung-yoon has been hanged whereas Dam has committed suicide but Wol-ryung, unable to tell her the truth, lied to her and did not tell her that he is a gumiho.
Wol-ryung decides to become human in order to be with Seo-hwa. To do so, he must live 100 days without showing his true form to a human, without taking a life, and must help anyone that needs aid. But if he fails, he'll lose any chance of ever becoming human, and will become a demon for the next thousand years. Wol-ryung successfully lives most of these days following these rules, but one day, Gwan-woong's men find Seo-hwa alone in the forest. Wol-ryung rushes to help her and reveals his true form, massacring the soldiers. Seo-hwa, horrified, leaves him, and he is later killed by the righteous soldier Dam Pyeong-joon (Jo Sung-ha), who had been told that the gumiho was murdering innocents. Seo-hwa soon discovers that she is with Wol-ryung's child and gives birth to a son. Realizing that the baby isn't a monster and regretting her betrayal to Wol-ryung, Seo-hwa entrusts the baby in the care of a monk, So-jung (Kim Hee-won). She then confronts Gwan-woong, but is killed.
The infant is adopted by nobleman Park Mu-sol (Um Hyo-sup) and grows up as Choi Kang-chi (Lee Seung-gi), ostensibly the son of Lord Park's servant Choi (Kim Dong-kyun), but raised as part of the Park family. Though Mu-sol's wife Lady Yoon (Kim Hee-jung) never warmed up to him, Lord Park loves Kang-chi like his own son, and he is close to the siblings Tae-seo (Yoo Yeon-seok) and Chung-jo (Lee Yu-bi), whom he loves even though she is betrothed to another. Kang-chi is notorious in the village as a troublemaker, but he is good-hearted and loyal, and beloved by the servants at the Hundred Year Inn, which the Park family runs. However, Jo Gwan-woong returns to the village. He believes the wealthy Lord Park has hidden treasure inside the inn and, in his scheme to take over it, Lord Park is killed defending Kang-chi. Gwan-woong throws Tae-seo and his mother in jail, and Chung-jo is sold as a gisaeng and convinced to use her beauty and wile to someday gain power and revenge. After Kang-chi promises Lady Yoon that he will take care of the Park siblings, she makes a futile attempt to stab Gwan-woong and is killed. Meanwhile, Gwan-woong becomes intrigued by Kang-chi and his seemingly superhuman strength.
In the meantime, Dam Yeo-wool (Bae Suzy) and Gon (Sung Joon) had been dispatched to the village by her father, now martial arts master Dam Pyeong-joon, to investigate a series of murders they suspect Gwan-woong is responsible for. Yeo-wool witnesses the events that befall the Park family, and instinctively helps Kang-chi when he is hunted by Gwan-woong's soldiers. During a fight, a soldier slices off Kang-chi's beaded bracelet, which was used to contain his powers. Kang-chi transforms into a half-human half-beast and easily defeats his enemies. So-jung later tells Kang-chi the truth about his origins, including the mystical and elusive Gu Family Book, which holds the secret to becoming fully human.
Kang-chi is taken under the wing of Yi Sun-sin (Yoo Dong-geun), a naval commander. Yi Sun-sin, Master Dam, and the late Lord Park were part of a secret group protecting the Joseon nation against foreign invasions. Yi places Kang-chi in the martial arts school run by Master Dam, there he is trained physically and mentally and learns to control his transformations. After discovering that under the man clothes there's a girl, Kang-chi gradually falls in love with Yeo-wool, and vice versa. But several obstacles remain in their way: the continuing villainy of Gwan-woong; the knowledge that it was Yeo-wool's father who killed Kang-chi's father; the reappearance of Wol-ryung who has turned into a soul-sucking demon with no memories who can only be killed by his son; and Sojung's warning that Yeo-wool is fated to die if she stays by the side of the man she meets beneath the blossoming peach tree under a crescent moon, which is none other than Kang-chi himself.
A martial arts instructor at her father's school, she is particularly skilled with a bow and arrow. When she was younger, Kang-chi saved her from being attacked by a dog and she never forgot about him. She then falls in love with him when they meet again. She also accepts Kang-chi for who he is inside even though he is a divine creature that many people are afraid of.
Kang-chi's first love. She is promised off to someone in marriage in order to sustain the family-owned-inn's longevity, hence the reason she originally first began spurning Kang-chi's open display of emotions towards her. After the family tragedy, she is forced to become a gisaeng. She makes a pact with Kang-chi that he should come and take her away from her current place once he has proven Lord Park's innocence.
A cool-headed, responsible scholar who helps his father Mu-sol run their inn. He comes under a hypnotism that makes him think Kang-chi murdered his father, testing the bonds of their friendship and brotherhood.
Kang-chi's father. He is the guardian spirit of Mt. Jiri. He's fond of traveling outside to see the world and during one such travels come across Seo Hwa and falls in love with her. They marry but he keeps his identity hidden until it's revealed and Seo hwa being too young and fearful betrays him with her impulsive actions. A betrayal which costed his 'life' since he lost the will to fight after finding out and doesn't fight back.
Not long after he's 'killed', his body 'dies' and stays still, hidden from everyone's eyes deep in the mountains for several years. After decades, he revives but as a '1000 year demon', striped of his 'guardian spirit' status. This happens due to him breaking the 100 days oath he took which he couldn't complete due to certain incidents.
Being a demon leads to him losing ability to heal or revive. The only thing left was hatred, anger, destruction and death causing him to go on a rampage, massacring several villages.
It also led to him having memory loses over time which he predicts that once he's lost it all, he truly would become an agent of chaos, a true demon. To prevent that, he tries to force his son through threats and intimidation in hopes of being put at peace since the only one capable of killing the current him was his own blood, his son Kang-chi.
It isn't until his wife, Seo-hwa sacrificed herself by committing suicide and apologize that he had a chance of becoming 'alive' again, not as a demon but as a mountain spirit.
Kang-chi's mother. She is the daughter of a nobleman who becomes a gisaeng after her father is framed for treason and killed but gets saved by Wol-ryung who protects and eventually marries her.
She becomes afraid and angry towards Wol-ryung after learning of his true identity and in impulsiveness, anger and fear, sells off his living place to soldiers, eventually 'killing' him. She tries to get rid of the child inside her womb in fear of it being a monster but fails. She eventually gives birth to the child(Kan-chi) and regrets her actions, asking him to not forgive her. After leaving her son in monk's care, she goes off to die as her punishment but survives and disappears.
She returns years later disguised as a Japanese lady Ja Hong-myung, and secretly plans her revenge against Jo Gwan-woong but finds her missing son and learns of her husband being alive but amnesic, having turned into a demon mainly due to her actions.
She stops Wol-ryung from causing any more havoc and apologizes for everything that happened. In the end she commits suicide in order to truly bring Wol-ryung back to life, not as a demon but as a guardian spirit. She dies in his hands after proclaiming her love for him along with one last apology.
A villainous nobleman. He lusted after Seo-hwa, then framed and killed her father, destroying many lives. He is now the mastermind behind a string of murders and a traitorous plan against Joseon, and hunts down Kang-chi.
A just and righteous nobleman, Lord Park is beloved by his countrymen, and is the father of Tae-seo and Chung-jo. When he finds the orphaned baby Kang-chi inside a basket floating in a river, he takes him in and becomes his surrogate father.
A former soldier, and now a martial arts master. Yeo-wool's father.
He's the one who leads the soldiers to Wol-ryung's residence since he deems the spirit too dangerous to be left alive. There due to Wol-ryung not fighting back, he manages to strike the spirit, dealing heavy injuries and 'killing' him.
He believes himself to be a part reason of Wol-ryung having turned into a demon and wants to stop the massacre however possible.
Strong ally of admiral Sun-sin and tough enemy to Gwan-woong after learning of his villainous actions.
Yoo Dong-geun as Yi Sun-sin – A patriotic naval commander with secret plans for building turtle ships that could aid in fighting against invading Japanese warships.
Kim Hee-won as So-jung – A Buddhist monk who was Wol-ryung's friend, and now watches over Kang-chi.
Lee Do-kyung as Teacher Gong-dal – A wise old teacher at Master Dam's martial arts school.
Jo Jae-yoon as Ma Bong-chul – A local gangster whose life is spared by Kang-chi, and becomes loyal to him.
Kim Dong-kyun as Choi – Lord Park's servant and Kang-chi's adoptive father.
Jin Kyung as Yeo-joo – Yeo-wool's teacher in domestic skills, who has a crush on Gon.
