No-one knows who he is or if he is even real.
He is a hacker, supposedly, but he could just as easily be a corporation or a computer virus. He has the power to hack people's minds via cybernetic implants, and he can control them and make them say things in their voice meaning he cannot be traced. Whoever he hacks gets a big smiley face plastered over their own, since he hacks everyone else in the room to see that happen.
Motoko Kusanagi has been chasing him for some time now.