I don't think the writer is a character of the story. But I suspect the writer will have a cameo appearance in the last episode.

I think Kyung's little brother is actually the writer. He keeps saying things that implies he knows how the story will end "you will definitely get your happy ending" and "dont change anything".

 I think he was upset at Dan Oh and Haru for betraying Kyung in the first joseon story and turning it into a tragedy, so he gave Dan Oh a heart condition in order to control her more and scare her. He also removed Haru completely from the new story as a punishment and to make sure he and Dan Oh never met again.

 Layhope:

you should mark this as kind of a spoiler because you actually might be right

Sorry about that, I fixed it!

 Lily:

I think Kyung's little brother is actually the writer. He keeps saying things that implies he knows how the story will end "you will definitely get your happy ending" and "dont change anything".

 I think he was upset at Dan Oh and Haru for betraying Kyung in the first joseon story and turning it into a tragedy, so he gave Dan Oh a heart condition in order to control her more and scare her. He also removed Haru completely from the new story as a punishment and to make sure he and Dan Oh never met again.

This!!

I think it’s obvious the writer favors Kyung more than anyone...and there’s only one person in this drama who worships the ground Kyung walks on.

 Lily:

I think Kyung's little brother is actually the writer. He keeps saying things that implies he knows how the story will end "you will definitely get your happy ending" and "dont change anything".

 I think he was upset at Dan Oh and Haru for betraying Kyung in the first joseon story and turning it into a tragedy, so he gave Dan Oh a heart condition in order to control her more and scare her. He also removed Haru completely from the new story as a punishment and to make sure he and Dan Oh never met again.

I also think that the writer could be him! But I didn't think about the reasons... Why he made himself as a younger brother?

 Cassandra89:

But in the same time... if the writer wants what is the best for Kyung, then why isn't Kyung the main lead? And why isn't he with the main female lead of the story? That would be perfect - as Dan Oh doesn't seem to be "good enough" for the writer to make her the main lead. Or is it some kind of revenge for the previous book where she was like a main female lead?

Kyung, Dan Oh and Haru were the main characters in "Trumpet Creeper". Just my guess but after seeing how much they suffered in the first story maybe he thought that they would be happier being extras and having more freedom?

I think the writer is never going to appear. I don’t think it’s who you guys think either. Why?  Cause that character made himself into an unfavorable position in Kyung’s life, also putting him with an abusive, bias father? If so, major plot holes.

On the other hand, I don’t think “Trumpet Creeper” was meant to be happy. Maybe the writers intended it to end in tragedy. The ‘Trumpet Creeper’ is special to Dan Oh and Haru. The name also implies cheating... I.e ‘creeper’ as in creeping over the wall, a play on the idiom “jumping over the wall” in Asian literature to mean cheating with the neighbour.