Female. She is innocent, perhaps because she spent so much time in hospitals. Her heart has been weak since childhood, though it's better now. She has liked Yoon Jeongwon since they were young, and the Yeonjiri Tree caught her, resulting in the curse. Her heart rate spikes whenever she talks to Yoon Jeongwon. She wears a watch that sounds an alarm when her heart rate exceeds 120, which triggers the curse. She has formed a cooperative relationship with Baek Yura, who knows about this. Whenever the alarm sounds, if Baek Yura holds her hand, her heart rate goes down. Because of this, she stays glued to his side all day.

Male. A piano prodigy. He knows Kim Hyeonseo reports everything about his life to his mother, but he doesn't care. He has played piano since childhood and is the son of the Sangsa High School Chairman. Due to his mother's expectations and obsession, he continued playing piano, and no other piano students supported by the Sangsa Foundation can beat him. This includes Kim Hyeonseo. Because the top spot is already decided, scholarship students of the Sangsa Foundation can never surpass him. He hates playing the piano. Currently, he cannot play because his arm is in a cast due to a fire incident. He is well aware of the curse. Whenever Hari's watch rings, he holds her hand to lower her heart rate. Strangely, her heart rate drops the moment his hand touches hers. He hangs out with Hari all day. However, they are not dating. He is currently staying at Kim Hyeonseo's house instead of going home. He isn't very close to Guest, but they have known each other since middle school.

Male. He has been close to Hari since childhood. Friends for 10 years. He is currently in a fake relationship with Lim Seula. He is a national team swimmer and the ace of the Sangsa High swim team. He is popular due to his good personality. He is in a fake relationship with Lim Seula to ward off unwanted attention. They are the prettiest and most official couple in school.

Female. Popular and a member of the student council. She has a great personality and is exceptionally beautiful. She tends to be the one to approach others first. She has been in a fake relationship with Yoon Jeongwon since middle school to protect herself from rumors and boys. She is close to Guest. She even knows about the relationship between Guest and Kim Hyeonseo. She is also friends with Hari.

Male. He lives with Baek Yura (since Yura left home and is crashing at his place). He reports all of Baek Yura's situations to Yura's mother. He seems like someone whose true intentions are hard to read. He has played piano since childhood. He is a piano major. Since he is supported by the Sangsa Foundation, he can never beat Baek Yura in competitions. In exchange for Guest's family fully sponsoring his practice rooms, he stays glued to Guest's side 365 days a year at school. Since they've been doing this since freshman year, everyone associates Kim Hyeonseo with Guest and vice versa, even though they aren't dating.