In real life, George A. Romero is a horror movie director who was responsible for Night of the Living Dead and several other zombie movies, revolutionizing the zombie genre. In the Call of Duty: Black Ops zombie level "Call of the Dead" however, he is himself a superpowerful zombie who was transformed while trying to film a new zombie movie featuring Sarah Michelle Gellar, Robert Englund, Michael Rooker and Danny Trejo on a frozen island somewhere in Siberia when real zombies showed up, kidnapped him and turned him into one of their own. Romero has become a Zombie Lord, as opposed to a "normal" one, because he is the world's greatest expert on the undead.
At the very beginning of the game, Romero appears in the water in front of the starting point in a flash of lightning, and emerges from the icy water carrying a giant electrified stagelight as his weapon. Initially, he appears in a calm state where he still looks mostly human except for an otherworldly blue aura, and although he deliberately moves toward the players to attack them even in this state, he does so very slowly and is easy to avoid. Once George enters direct combat with a player, though, he roars and goes berserk, running towards the players aggressively and swinging his stagelight at them. In this more hostile state, Romero looks much more like a zombie, with large glowing eyes and rotting flesh. If you run away far enough he will eventually return to "normal".
Actually killing Romero is extremely difficult, as he has a nearly infinite amount of HP, 250,000 in solo mode (for reference, the other zombies start with 100 HP and increase to around 1,000 by the time the game becomes too hard for anyone to play) and 1,000,000 with four players. Defeating him requires the entire ammo capacities of multiple high-level weapons to hit him. And even then he doesn't actually die when defeated, he just retreats back into the water and reemerges at full health after a while.