Yoon Ji Hoo is a calm and soft-spoken boy who likes to play the violin, piano, guitar and conduct an orchestra. Seo Hyeon was Ji Hoo's first love, after she helped him overcome trauma after the death of his parents when he was little. He attempts to fight for Seo Hyeon's love by following her to France. But then he realizes his love for Seo Hyeon was just worshipping her.When he comes back, he realized that he had fallen for Jan Di. However, Jun Pyo has already announced his love of Jan Di. Ji Hoo decides not to betray his friend by trying to fight for Jan Di. He is a grandson of Korea's former president.
Yoon Ji Hoo is part of the famous group F4. He is admired for his musical talent in which he knows how to play many instruments. Althouh he is of the feared F4, he does not get involve in their bullying schemes and prefers to play his violin. He saved Geum Jan Di from some of the bullies when she entered Shinwa High School.
Jan Di is in total awe of Ji Hoo who came to her rescue more than once. At first, Ji Hoo does not reciprocate these feelings as he still love Seo Hyeon, although his love for her is unrequited. He loves Seo Hyeon from that fact that when they were young, she took care of him and is a substitute for his mother. Both his parents died in a car accident in which Ji Hoo blamed himself as he covered his father's eyes whilst his father was driving.
As the drama progress, Ji Hoo discovers that he actually has feelings for Jan Di only to find out he is not the only one as his friend Goo Jun Pyo also likes her. The relationship between Jun Pyo and Ji Hoo becomes intense after Ji Hoo kissed Jan Di. They even engage in a fierce battle that resulted in the revival of their friendship. He always remain by Jan Di side despite the fact that she has fallen fo Jun Pyo.
After his parent's death, he refused to talk to his grandfather, the ex-president of Korea turned doctor. With some encouragement for Jan Di, their relationship improves. Towards the end, after this grandfather's death, Ji Hoo decides to pursue a career in becoming a doctor.