Jo is the Orion/the hunter and he will be the one to jump into the dangerous life. The story will not come to an end without the involvement of Ian, Jo and TJ. Also, there's more to Jo than what he and the readers had presumed about himself. Everyone has a dark side. It’s just that for people like TJ, it comes out easy as if second nature & for people like Jo, it stays repressed until it hits that breaking point & can no longer stay hidden. So sooner or later, that side will come out of Jo. It’s just a matter of time. At first Jo was reluctant to cut open than rabbit but later he successfully did when he was only 10 years old. He used to be a NFL player. They naturally possesses an aggressive demeanour. He didn't face any situation that challenge him to see who he truly is and what he's capable of since he leads a normal and happy life. I think Jo's character highlights and self discovery will be when he's next to Jamie (main villain). Remember, Chiwoon thought Jamie's men are circling Ian but Jo they are interested in. Jamie wants Jo.
You might be right, but I think that Ian is interested in Jo mainly because he is so different from the gangsters he grew up and lived with. Ian sees a different life while with Jo. If Jo becomes turns to the dark side, then, there won’t be much difference between him and TJ anymore, so he will have even less chances of being the endgame.
One becoming dark doesn't necessarily mean they will become evil. Him becoming dark is necessary for his character development. We have yin and yang energy in ourselves. We need both energy. In order for the light to brightly shine, the darkness must be present and it's all about how he balance both energy within himself. I'm telling you, he's going the get fucked up really bad and we will witness his breaking point. It's really going to get raw because he is someone that has never experienced any of this things before in his life. That's how he can rival TJ.
There is no room for Jo's plot to really grow unless he join the gang fights or politics. He'll always be that outsider looking in, while Ian and TJ are playing the real game. Maybe Ian sees Jo as a fantasy/escapism? to have a life that's a bit normal with someone that's more normal. All that is just wishful thinking. One can never escape the thug life once you're in.