@ceneoss
He did reject him, but by SJ being a public figure, it'd be easy to catch his departure. He can't stop him from coming of his own will either.
In a sense, it's like; "I'm not following you, I just happen to be have the same destination."
There's no "same destination", Japan should be worrying about getting people out of the country not in, so he wouldn't be able to enter. And he wouldn't be able to access the battle zone anyway, no way the Japanese guard would let him anywhere close to that gate.
Besides Jin-Woo was officially called by an alliance between Japan and Korea's government so I don't think it would be hard for him to stop the brat's entry.
I'll be legit mad if next chapter this dude is following Jin-Woo in the battlefield. There's no reason for that to happen.