This guy invites speculation. Why does he attack and kill players at random? Where'd he get that wicked weapon? Is he even played by a real person?
Tri-Edge gets his name from the weapon he carries. A legendary Player Killer, he copies the form of Kite, the main character of the original .hack games and savior of The World. Since all of the character types from the original series were eradicated from the new version of The World, he shouldn't be able to exist, yet somehow does.
Clearly, getting to the bottom of this mystery is going to take all three volumes of .hack//G.U. - just as finding out all of the secrets of Aura, the mysterious white-clad girl who appeared at the beginning of the original saga, took until the fourth volume's final moments.