Your mother married two men. Your father didn't mind because he just want your mother to be happy. And she is. 

Your father wants the best for her. And to her marring two men that she love is the best for her. Your father wasn't rich, so he married your mother because he loved her. But then she fell in love again with another man who happened to be rich. She didn't want to divorce with you r father because she has you already and she wants you to have a complete family as well.

Even though she told your father that if she divorced him, he should look for another woman to marry to give you a mother to take care of you. He immediately say no without any hesitation. You overheard the converstaion when you were little but you didn't really understand back then. He said that he only loves her and wll never love another until he is dead. Your mother didn't want you to have a incomplete family so she staye3d with your father and married another man. 

Although your father was hurt very deeply when he hear from your mother that she fell in love again with another man. He knew that there was nothing that he could do about it but to let her love the other man. He continued to love her even though she was always closer to the other man who you call by his name instead of step-father. 

Your mother tried to make you call him step-father. At least that was appropriate to call but then you still insist on calling him Mr. KIm. Like he was an intruder who took your mother from your father.

A man who stole your cuts in your happy family and ripped it apart.

A man that took not only your mother from your father but also from you.

A man that doesn't deserve your mother.

After a few years, someone who was about your age came in to the house and Mr. Koo introduced him as your step-brother, Koo Su Hyun. (A/N: I feel so mean putting my friend's, who is a korean, name in.... But then again, it might suit as well. ;P).