Hopefully Grens will have more relevance in the future but I agree hes quite a pointless character I guess hes simply just to show the tragic backstory of Shin and what he could have become? Probably not that deep though
I would argue that Gren brings out an interesting side to Shin's personality.
=> Gren was the reason we - and thus Jake - could realize how much family & emotional links meant to him. Before, Shin seemed pretty detached from reality, a "whatever happens, happens" guy, but Gren is the proof that it is much deeper than that.
Plus, he also is an accurate depiction of what addiction to drugs can look like.
It is sad for him to be centered around Shin, but some people irl are also this way. All he needs is time & better context for him to evolve as a person. :/
This is one of the veryyyy few manhwas where the "uke" (this word simply does not fit him at all) is actually portrayed in a more realistic manner. I love buff and strong bottoms but usually you can tell that the bottom is meant to be weaker and smaller than the top or becomes a pussy in front of the seme. Shin just doesn't feel like that at all. He's strong, stands up for himself and honestly if they switched positions I wouldn't even be surprised lol.
This is also because Jake also seems more realistic and open-minded. He's intimidating and clearly more powerful and yet Shin doesn't feel weaker next to him. They're like on the same level and have strong points in their own respective areas.
I don't want to say that their relationship is exactly healthy but it's not toxic either. Considering the context (gang life, drugs etc.) their relationship could have been...a lot worse and yet something about their relationship is so realistic and seeing it develope is interesting because it's incredibly subtle.
Unlike in most manhwas love doesn't appear out of nowhere, and most of the time creeps up on you which is what I think is happening here. It's super cool.
I'm excited to see what happens next and hopefully it won't be ruined like many other such cases. The only complaint I have is that Gren basically has no point in even existing in this plot.