![](https://pic1.mangapicgallery.com/r/avatar/54/u594468_50x50.jpg?2)
I originally assume that also until one of the chapter, they explain that "that guy" is referred to as "mad", the beginning and end of neonmask channel. A person who was able to beat up our mc miserably, it could be that they were talking about our my being mentally beat up by hiss split personality to the point he loses control and want to kill people. That would be a cool plot but something tells me that ain't it for some reason....
As someone who read a lot of action novels and manga, I kinda think "That Guy" is referring to Sungbin's split personality. Maybe he had it due to trauma caused after the death of his sister. The triggering factor of this personality is probably depend on overwhelming emotions. I might be wrong but I'm gonna stand on it until the author reveals the truth of Mad.