He doesn't have a split personality. He's Narye and has the memories of his past life, he reunited with mc because he bind his soul to him.
He's the same creepy guy as before, but because he knew that his actions in the previous life drove mc to suicide, he chose a different approach this time by being a good and caring friend to let mc relies on him.
But he's pretty much the same, he isolated mc in school (mc was already not socialble, but he actually made sure behind mc's back that nobody will approach him) to be sure that mc only relies on him. You can actually see that when he rejected the school meeting for mc in the car, and mc was shocked because being that controling its the same as Narye.
To made MC totally fall in love with him, he hired an actor to act like Narye so that Mc will abandon his doubts against him and see him as a savior against Narye.
Because mc wanted to go study abroad he also asked the actor to kidnapp him and to take him to the airport so that mc would develop an airport trauma... you see? He's just Narye lol.
But because he knows that as soon as mc will notice he's narye he will run away, he has to be always carefull to not let his true personality show up in front of mc. All his demeanor in front of mc are just an act. His only one and true personality is being Narye the psychopath.
Omg, I expected the black haired guy to be a good person in his next life, even if he got reincarnated, but turns out he still has this fucking creepy and scary demeanor, and to add it up, it seems like he has a split personality, I'm so confused HELP