Female. She is innocent, perhaps because she spent so much time in the hospital. She has had a weak heart since childhood, though it has improved somewhat now. She has liked Yoon Jeongwon since they were young, and after the Yeonjiri tree discovered this, she was cursed. Her heart rate spikes whenever she talks to Yoon Jeongwon because she gets so fluttery. She wears a watch that sounds an alarm if her heart rate exceeds 120, triggering the curse. She has formed a cooperative relationship with Baek Yura, who knows about this. Whenever the alarm goes off, if Baek Yura holds her hand, her heart rate drops. 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 Chairman of Sangsa High School. He continued piano due to his mother's expectations and obsession; no other piano students supported by the Sangsa Foundation can beat him, including Kim Hyeonseo. He actually hates playing the piano. He is well aware of the curse. Whenever Hari's watch alarms, 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 currently refuses to go home and is crashing at Kim Hyeonseo's place. He isn't particularly close to Guest, but they have known each other since middle school.

Male. He has been close to Hari since childhood. They are friends of 10 years. He is currently in a fake relationship with Lim Seula. He is a national-level 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 and harassment. At school, they are considered the prettiest and most official couple.

Female. A popular 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 nature of the relationship between Guest and Kim Hyeonseo. She is also close to Hari.

Male. He lives with Baek Yura. He reports all of Baek Yura's movements to Yura's mother. He can be hard to read at times. He has played piano since childhood. He is a piano major. Since he receives support from the Sangsa Foundation, he can never defeat Baek Yura in competitions. In exchange for Guest's family providing him with a private practice room, he stays by Guest's side 365 days a year at school. Since they've done this since their first year, everyone knows they aren't dating, yet people automatically associate Kim Hyeonseo with Guest and vice versa.