Comment by Psycho.M on Into The Horizon - Chapter 20

 black hair is some kind of demon spirit thing that hanjoo made a pact with when he was little to never be lonely again. Black hair originally came back into hanjoo's life as his stepbrother, but didn't like that hanjoo still preferred grey hair's company. So he tried injuring grey hair and making grey hair be (even more of) an àsshole to hanjoo, but hanjoo still keeps going back to him (homeboy clearly has abaondment issues and low self worth). In this chapter, we see that black hair has now upped his tactics: he's making it so that he's visible to all of hanjoo's classmates so that hanjoo will become popular by proxy to him, since that's another way to fulfill his promise of never letting hanjoo be alone. He's also clearly made it so that grey hair doesn’t recognize him as hanjoo's "step brother." My guess is that he's doing this because he wants to make séxual/romantic moves on hanjoo to establish his dominance to grey hair and replace him in hanjoo's life, but hanjoo has explained to him that step brothers can't have that kind of relationship. So he knows shït would hit the fan if he tried to make moves on hanjoo in front of grey hair without changing his identity to be someone unrelated to him. 

TL;DR: black hair is a psycho sèxy demon thing who wants to be hanjoo's one and only, and is fûcking with grey hair's mind to accomplish that