The Complete Nut is Lester in a cow suit, and can only be seen through the Command.txt exploit. He will give you a quest that replaces the Rune Bomb quest (don't worry, you'll still be able to get in the Festering Nest). At the end of the quest you get a powerful suit of armor, called the Bovine Plate.
He is also one of the few NPCs who changes appearance in the game. After retrieving his brown (moose) suit back from level four of the Festering Nest, he will put on that suit instead. Note that the grey (donkey) suit will not suffice; bringing it to the Complete Nut will result in him sending player back into the Nest.
Hellfire's manual gives a hint about the existence of the Complete Nut: it is mentioned that the townsfolk were worried about Lester's sanity as he had taken to talking to the cows for hours.