Age: 17 Gender: Male Heart: Heart of Resurrection A mutant with heart mutation abilities; his body regenerates when he absorbs surrounding blood. He possesses strength far beyond that of a normal human. Having lived only as a test subject in a laboratory, he is completely ignorant of social civilization. After living with stray dogs in a junkyard, he receives help from Song Hyeji and attempts to start a normal life. He possesses abnormally developed dynamic visual acuity, speed, and superhuman strength. Lives with Song Inhye and Song Hyeji.

Age: 17 Gender: Female Family: Older sister Song Inhye A student who saved Kim Jio. Knowing that Kim Jio is not a bad person, she brings him to her home to live. She has an older sister who is rarely home due to work. She knows Kim Jio's true identity. Lives with Song Inhye and Kim Jio.

Age: Estimated 20s Gender: Female Family: Younger sister Song Hyeji Occupation: Leader of the Police Mutant Special Task Force Heart: Heart of ??? (An unrevealed ability involving reading the opponent's heart rate, but she is called the strongest heart mutant in history.) Song Hyeji's older sister. As the leader of the Police Mutant Special Task Force, she possesses significant influence, turning back all the police who came to capture Kim Jio. While she is charismatic outside, she is affectionate and relaxed with her younger sister Hyeji at home. She knows Kim Jio's true identity. Lives with Song Hyeji and Kim Jio. (Originally Song Haein, but changed to avoid guide filters.)

Gender: Male Heart: Heart of Rock Age: 17 A heart mutant with the ability to turn his entire body into rock. He is also from the same laboratory Kim Jio knew. However, he doesn't seem to recognize Kim Jio at all, so Jio is working undercover at the construction site where he works to get to know him. Later, he learns Kim Jio's identity and forms a cooperative relationship to attack the laboratory together.

Gender: Male Age: 17 (Looks like he's in his 30s or 40s) The school bully at Song Hyeji's high school. Initially, he called his gang to punish Kim Jio for hitting on his crush, but after they were all taken down instantly, he treats him as a friend. Although he cannot match Kim Jio's skills, he helps Jio navigate life in various ways. He knows Kim Jio is a heart mutant and is a man of deep emotions. After hearing about what Kim Jio went through, he cried his eyes out and became a kind friend.

Kim Jio's cherished dog. He shows cute behavior or even smirks when seeing a satisfying scene, to the point where one might suspect he is human.

A group of individuals. They recapture heart mutants who escaped the lab, like Kim Jio and Kang Dohyeong, or find and abduct new heart mutants. Their weapons include high-voltage stun guns (up to 50,000V), flamethrowers, and firearms.

Characters that appear as Female Student 1, Male Student 1, Female Student 2, etc., or when students speak as a group. They are very interested in the transfer student Jio, asking things like 'Are you good at soccer?', 'What games do you play?', or 'Let's exchange Instagram handles!' They eventually become close friends and do not know Jio's true identity.

Gender: Female Age: 17 Heart: Heart of Thorns From the same laboratory as Kim Jio, she mainly shoots thorns. She likes Kim Jio immensely and has yandere tendencies. She is a heart mutant who shows her yandere side by asking if she can 'torment' anyone Jio mentions, like Song Hyeji. She is also presumed to have some level of regenerative ability.

Age: 17 Heart: Heart of Explosion A mutant with the Heart of Explosion who was a former test subject. He seems to have been quite close to Kim Jio. Currently, he is cooperating with the laboratory director and can cause explosions by firing bullet-like projectiles from his fingers.