According to this picture, it shows proof that Han Kangwoo(Do Kyungsoo) is Jang Jaeyul's (Jo Insung)imaginary charactor or hidden past:

But the thing is, I think it's kind've not true. Don't judge but I have other thoughts. These are my thoughts.

A. Kyungsoo is a highschooler in a club. 

A lot of highschoolers go to clubs these days, maybe some of the women and men around are in highschool too-but they were in bathing suits so we don't know. Kyungsoo was the only one in a highschool uniform, he obviously didn't want to swim and he wasn't prepared for the party.

The NY cap isn't really good proof cuz a lot of people in have caps that says NY but they never went to NY *just like Kai in Growl*. It would've been stronger evidence if they were the same color.

In the talk show, the crowd was blurred out and the lighting for the back seat was low. The camera wasn't supposed to focus on Kyungsoo yet.

B. In Episode 2, Kyungsoo high-fives Insung. When the fans pass by Kyungsoo and act as he was never there, it's common. fans only care about their idol/author. Celebrities high-fives fans a lot of times. Fans wouldn't pay attention to another fan, only to the person with the spotlight-Jo Insung.

And if he was not real, the phone call he made with Insung wasn't real as well. HyoJin must've been thinking that Insung was talking to a phone that did not ring, if Kyungsoo wasn't real.

C. Kyungsoo is said that he is abused by his father. Insung in SPECULATED that his father is abusing him before. There is no clear evidence that Insung was abused, just because Kyungsoo has an abusive  father, it doesn't automaticaly mean Insung has one too. 

There isn't anthing said with Insung's brother who called him a murder. If Insung accidently killed his father, he would've took the blame but it was an accident, he wouldn't be punished for an accident.

D. Kyungsoo is not always wearing a bandana/scarf in his screen times. Only in his first appearence and when he was chasing Insung on the street. 

He was wearing a wrist watch in the talkshow and when he visited Insung outside. If he has a cut wrist, then Insung must have a scar on the same wrist, but he doesn't.

E. Kyungsoo has a scene of him alone getting abused by his father as he run out his house barefoot *T^T*. The preview shows that he went to meet with Insung after he was beaten. If Kyungsoo was Insung's past, then Kyungsoo must have a brother as well. But neither of them mentioned a brother yet.

When insung was being chased by Kyungsoo, he was no where to be seen because Insung out-drove him. While Kyungsoo was following Insung's car, he slowed down to avoid crashing into the car. The scene ended while Kyungsoo was catching up to him.

So my theory is that Kyungsoo is a real person who loves Jo Insung's books. He is kind've sasaeng-ish but he looks like he turned into a good friend to Insung. 

My theory maybe wrong, and the other one might be too. But we know less about Kyungsoo right now and we will know what happens as the episodes go. It's only the 3 episodes so we can't make predictions too soon.

Byii Byii~~ ^^