That said, what magnificent characters and character dynamics in this manhwa! Gren's presence does give this story some interesting depth
I 100% agree. Gren hasn't seen Shin in ~20 years, then his strung out ass pops up out of nowhere and plays the extremely tenuous "family card" to blackmail Shin into helping him fix the life that he's gone and royally f*cked up all on his own. I hope that conversation with the homeless man knocked some actual sense into Gren (to get his life back on track). Addiction sucks, but it's not something that can't be overcome with hard work and determination.
How he reacts to what the homeless man said to him will tell us everything we need to know about Gren
Gren is emotionally blackmailing Shin and it's definitely affecting Shin. For those seeing Gren as a helpless child by no fault of his own because of his shitty upbringing, and justifying him shooting Jake because Jake is known as a ruthless killer, let's pretend that Gren himself didn't just ruthlessly and with no remorse, kill off his own gang while they were lying helpless in the streets. The first thing he does when he meets Shin? Tries to recruit him into a gang. Now he wants to Shin to leave his dream job, run away with him and live on streets. Also, the reason he was in jail in the first place was for robbery and blackmail. Shin's good friend Hosung was doing a smart thing by warning Shin not to be fooled into thinking Gren was same little boy that he remembers from his past.
Gren's definitely a tragic figure, but he's clearly not even attempting to veer from his destructive path right now. Shin has to do what's best for himself, and that means staying clear of Gren. Sadly, you can't do anything for addicts. They have to do it for themselves or they won't succeed. It's not up to, nor is it on Shin, to pull Gren out of his own mess