In Sung Yoon

In Sung is the first born son (second child) of the YK line, the third largest group in the financial world. He is Queen's little brother and the heir of the YK group. When he first met Jae Gu, he challenged him into a fight were he lost telling him he is worthy of his sister. He cares about his sister a lot and knows she has feelings for Jae Gu but she's hiding them. He meddles in and tries to help them both because he wants his sister to be happy. He is spoiled and acts like a little kid sometimes. He buys anything standing in his way.