I'm not sure if he is the begger or not, because how can he knows many things about their ...

I'm not sure if he is the begger or not, because how can he knows many things about their childhood; so I don’t think he is the begger. And it could be that the childhood friend has some mental problem, he was living a fucking life only being obsessed with woojin, we can’t say he is normal!!! I’m sure he drugs him because he douse the believe in him and he wants to be sure that he will stay with him!!
But this is only my theory