Spoilers

For those unaware, Kibum is the younger brother of the man who Woohyun owed money to in Part 1.  After discovering that his brother had died, Kibum took over the business to track down all those that owed them money and make them pay.  He finds Woohyun but decides to play with him so he poses as a cafe owner.  His bodyguard, Jonghyun works as his assistant at the cafe.

- The person in the foreword is Kim Kibum, looking at a photo of Woohyun.

- At the end of chapter 1, the movers are reporting to Kibum about the whereabouts of Woohyun.

- The man who attacked Woohyun at the end of chapter 6 is that Mr Kang from Part 1 who was fighting with Sunggyu over Woohyun at the club.

- In chapter 7 when Jonghyun pursued Mr Kang and came back to say that he lost track of him, he was lying.  He captured the man and later killed him so that they can frame the man for what happened to Woohyun.

- The two people at the end of chapter 7 are Myungsoo and Sungyeol.  Sungyeol is still trying to capture Myungsoo to complete his mission.  They end up battling it out, resulting in them both getting injured as revealed in the later chapters.

- In chapter 9, the injured person Sunggyu sees is Sungyeol.  He was wounded after his encounter with Myungsoo.

- In chapter 11, the person who actually kidnapped Woohyun is Kibum.  Woohyun doesn't know who it is because he was blindfolded.  After Woohyun was released, he meets Kibum who pretends to come to his aid.

- At the end of chapter 12, the body floating in the river is Mr Kang.

- In chapter 14, the person who visited Sunggyu in the hospital is Myungsoo.

- At the end of chapter 15, the person Hoya hands the money to, you should be able to work out by the end of the story.

- In chapter 17, Sunggyu meets up with Myungsoo, wanting to confront him about what happened to Woohyun but Myungsoo denies doing it.  The person who attacked Myungsoo from behind is Sungyeol.  Sunggyu had made a deal with Sungyeol to help him complete his mission in capturing his brother but he doesn't want his brother harmed.

- In chapter 19, guess who the mask person is.  You need to read the last chapter.  Plenty of hints there.

- In the last chapter, you find out that Sunggyu was knocked out by Hoya when he tried to go save Woohyun and Hoya says that he didn't do anything.  Someone got paid for doing a job well done.  About Hoya and Sunggyu's past, I left some clues throughout the story. 

So, hope you all figured it out now.