Kim Hye Sung is the older brother of Hye Shin Kim. He is a former high school student who was bullied into becoming a recluse, believing his bullies were targeting his family. He stays in his room searching his bullies on social networking sites and believes that they all have perfect lives. With the help of Jae Gu Song and Kim Ho Pae he learns that this is not true and becomes less of a recluse over time.
Hye Sung is a very slim, tall young man with dark brown hair and lilac eyes. One of his most noticeable features is the shadow below each of his eyes, a result of him always being on a computer. Hye Sung is always shown to be wearing blue jean trousers and a long-sleeved, white shirt. He almost never wears shoes or socks, preferring to go barefoot around his house, otherwise he wears slippers or loose wearing shoes. Hye Sung is also suggested to be homosexual.
When Jae Gu bring him into Wild's High to wash himself up, he is shown to be a handsome man with very delicate features that cause Kim Joon Gu to have a crush on him.
Hye Sung is depicted as being someone with severe social anxieties, remaining locked in his room for extensive periods of time and refuses to eat food that is left at the door for him by his mother. He watches out for his bullies through the window and social networking sites making sure that they stay away from him and his family. Believing they have the perfect life, he doubts his own life and self doubts himself constantly.
Being in a constant state of fear, he dreads any male that visits his house fearing Song Jae Gu. when he sees him with his sister. Believing him to be someone sent by his bullies. After he is shown the life his bullies are living by Hyung-nim he becomes less of a recluse, and shows a happier side to himself. He is shown to be proud of his younger sister and is more than caring for his family, protecting them to the point of sacrificing his own safety and happiness for them to be kept safe from his bullies.
He is forgiving towards his bullies and is willing to help those who require help. He is also apologetic to all those he feels he has wronged.
During his high school years he was a more open and social man. He specialized in Kickboxing and trained alongside Hye Shin. He often read books during his school time and ignored the bullies until one day he asked them to stop believing everyone should get along. This angered his bullies who began to terrorize him every day, even though each day he would win against them.At one point him and Hye Shin's mother had a vase dropped on their head believing his bullies targeted her, he became fearful for his families safety and became a recluse.