A person who is not important enough for anyone to remember their name. That guy is most likely a loner or socially awkward.
Who the hell is that guy and why should I even care?
That guy is a nickname for someone who is generally annoying and always sort of a dick.
Person 1: Wait, is that..?
Person 2: Ugh, it's THAT guy.
that guy is a person who is very cool and is just that guy
good luck: have you seen campbell walking around he's so cool he's that guy
campbell: im that guy