Yu Dayun is the younger sister of Yu Ijin.
Dayun is small, black-haired girl in her teens who is considered to be one of the prettiest girls in the school and she was popular in her younger years due to her intellect and looks.
When her parents died and her brother, Ijin, went missing, she started living with her grandfather. She was the most popular girl in middle school, which led to many guys asking her out. Because of this, Heejin Kim and other jealous girls in her school started bullying her, forcing her to buy snacks for them and taking any opportunity to physically abuse her. Eventually, she was reunited with her brother, who took care of the bullies for her.
Yu Ijin (Older Brother)
Ijin, being her long lost brother, she treasures with all her heart. She fears that she is the cause of him getting into all these fights, and tries to keep him out of trouble. At first, the two had a slightly awkward relationship, with both of them not knowing how to talk to the other, but eventually they got used to it. She often cooks him breakfast. Yu Dayun is unaware of Ijin's past as a mercenary.
Dayun is a fan of Cha Seo-Ha, the celebrity her brother was protecting as a bodyguard. Ijin approached Cha Seo-Ha for her autograph, as a birthday present for Dayun. Surprisingly, the actress visited Shinan High school and personally greeted Dayun, even taking a selfie with her.
Dayun loves her grandfather dearly, even refusing a new smartphone because it would cause him more trouble. She looks up to him, copying his recipes and attempting them. She appreciates all he does for her and tries her best to lighten his burden.
Heejin was one of Dayun’s bullies who has a long-held grudge against her for losing the affections of her crush to Dayun, even though it was not her fault. Heejin constantly berates her, even going as far as to physically attack her. Although Dayun does not care for Heejin, she stopped Ijin before he attacked her because it would weigh on her conscience. It is a mystery on what exactly Ijin did to make Kim Heejin and the bullies disappear. Nevertheless, thanks to Ijin, Yu Dayun stopped being a victim.
Shin Yuna is Yu Dayun's senior who used to take care of her before leaving abroad for two years. Even after returning, the two are good friends.