Yoo Da-in is the main character in story and at the start he is a really "pretty" guy so he was often mistaken for a girl. Shortly into the manga he is changed into a vampire and turns into a girl!
(In the English version, they renamed 'Yoo Da-In' to 'Gene Young' and 'Yoo Da-Young' to 'Jean Young.')
He is medium high boy with brown short hair and beautiful green eyes. As a male, he is often mistaken as a woman because of his appearance. When he is 'Medusa' his hair grows past his waist. And though he retains most of his former looks as a guy, he developments more feminine characteristic, such as curves. Da In was also warned against the effects of his eyes, something that was again repeated to Seo brothers against Medusa. For this reason, he donned glasses for much of the book, deflecting the hormones emitted when he was not transformed into Medusa.
He's an orphan and during his childhood he was often teased by others for looking like a girl, but now he usually attacks them if they think he's a girl. When he was 12, he met Seo Wool and became good friends. The manga starts out when he goes to an all-boy high at 18 years old. He was the leader (jjang) of a gang since he was the strongest.
One day, his friend, Seo Wool, asks him to pretend to be his girlfriend in an attempt stop another girl from following him. Though displeased with the suggestion, Yoo Da-In agrees to do this and gets stuck wearing a wig; under the alias, Da-Young. Yoo Da stops by Seo Wools school and looks around in awe. However, before he meets Seo Wool, he sees a male and female student arguing. He watches behind a tree as the girl continues to beg for the boy's affections. He continues to push her away until he finally grabs her about the neck and chokes her. In an attempt to save her, Yoo Da attacks the boy but to little effect. Seeing how the boy is uninjured, Yoo Da flees with the girl. But she later slaps him and runs away as a baffled Yoo Da watches her departing back. Later that day, Yoo Da's gang was planning a group battle. But before they left, the others asked him to dress up as a girl for confidence. However, when they got to the base of the rival gang, they were easily defeated. Because of the wig, the rival gang members are convinced Yoo Da is a girl and brings him to their leader, Seo Ji-hoo. He is helpless to fight against Seo Ji-hoo, who declares he'll make Yoo Da his bride. But just as Ji-hoo tries to bite him, his brother, Seo Kng-hoo interrupts. Yoo Da recognizes Seo Kang-hoo as the guy he kicked in the garden.
Fighting ensues and Kang-hoo is stabbed. Yoo Da holds him and they escape. Kang-hoo urges Yoo Da to run away but concerned, he does not and is bitten by Kang -hoo. He staggers away and dies on the street as fearful pedestrians look on. It is later discovered that he has turned into a vampire after being bitten by the vampire elitest, Seo Kang-hoo. Yoo Da's unusual characteristic of transforming into a woman, drastic behavioral change and the rarity of his survival into a vampire, led him to become deeply entangled into vampire politics, community, and history.
If a girl is bitten by a male vampire and survives, she becomes his bride.
This is orginally the primary motivator of Seo Ji-hoo in pursuing Yoo Da as his own. When they met again at the Seo residence, Ji-hoo attempts to take him away. But when he is finally convinced that Yoo Da, no longer female at the moment, was a man, he tries to kill him. Yoo Da fights back and drinks his blood, before collapsing and seemingly dying. During his treatment, an appointed family doctor explains that Da-in is, in fact, a girl. As he continues to recover, Kang-hoo pledges to take responcibility for him and erases him from the memories of all his acquaintances and friends.
During his time as a vampire he has many different kinds of run ins and is often in danger. These dangers include conflicts with Suh Hwa and her brother Suh Ryung. He is kidnapped by Yun Chae-Seong and even used by his mother, Meduza. It is hard enough being a teenager and on top of that a teenage vampire with gender problems.
Yoo Da-In was truly meant to be a girl, but her mother transformed her before she left the womb with the condition that as long as she didn't get bitten in her human boy state, she wouldn't transform into her true form as an alluring female vampire whom attracted many vampires.
Medusa: is Da-in's mother. Her spirit inhabits her body. She's responsible for transforming Da-in into a human male right before birth.
Seo Kang-hoo: Is the man who turned Da-In into a vampire. Throughout the enitre manga their relationship grows from Kang-hoo protecting Da-In into a romantic relationship. The character transforms from a calm and seemingly cold guy to a passionate and impulsive man. When Da-In is unable to bind himself to Seo Wool for life, the pair attempt to run away. But Kang-hoo disappears at sea and once he reappears, he is somehow able to remember Da-In despite everything.
Seo Ji-hoo: is the man who wants to turn Da-in into a vampire to become his bride, but was interrupted by Kang-hoo. Initially, Ji-hoo hates Da-in because Da-in as a male is no use to him. When Medusa first appears in front of Ji-hoo, he hates her as well. However, after spending time with Medusa without Da-in's consciousness being suppressed, he later develops a crush on her. He also thaws towards the male Da-in, who he treats as a servant. When Kang-hoo goes was missing Ji-hoo and Da-in make a promise: If Da-in became a vampire again he would marry Ji-hoo, but Da-in broke the promise.