"That guy" is a nameless character from the video SMOSH FOUND DEAD.
"That guy" plays a major role in 'SMOSH FOUND DEAD' as the sole antagonist. He is the owner of a restaurant, as well as all the employees who work there and the fake police. As the video, progresses, Ian and Anthony know that all people in the restaurant, as well as on their recent Twitter questions, are the same guy.
"That guy" seems to have a vast amount of supernatural powers, including the abillity to teleport at will, appear in different costumes with almost no time having passed, and even manages to kill Ian and Anthony without being seen.