"Can you really beleive we are alone? That Mainframe is the only system in cyberspace? I think not."
-Welman's speech (in Dot's flashback).
This is Enzo and Dot's mysterious father--the one that deleted all the sprites in Mainframe. He is the professor of Mainframe, before Bob ever arrived in the city, yet his experiment on the gateway machine was deadly. After 3 seasons, Professor Welman Matrix finally returns...
Welman's sprite code is within Nibbles, the one Mb called father (and rightfully so as Welman's gateway machine created him as well as Hex). Hex, who is on the side of Bob, helps Dot (although out of Dot's force) to bring back Welman from the pile of nulls that talked resulting in a 'jelly Welly' (right). However the null version of Welman can only exist in the presence of Hexadecimal, the benign virus who sacrifaces herself...
Before leaving, Hex gave Enzo the code to revive Welman's life (although she had never told Enzo). Enzo later discovered her little gift when giving Nibbles a touch, turning Welman from his Nibbles persona to himself. We now have yet another Matrix on the side of the good.