They are polar oposite while still share some similarities. While Jinwoo grew up to be an admirable young doctor, Minho inheritted his dad underground business. 

The rough childhood brought them together and also push them away from each other. Jinwoo was there when little Minho needs him and Minho was there when Jinwoo needs him. 

They don't owe each other nothing. All they share was those little scerets in the past. 

What is our relationship? It is when years later, just a word from you, I'm willing to give up my life, for you.