We don't need Users. We've advanced. They're superfluous.
The Master Control Program, better known as the MCP, was once the highest-ranking program in the Hollow Bastion OS. He appears in Kingdom Hearts II as a major antagonist bent on taking over the User world.
The Master Control Program appears in two forms over the course of Kingdom Hearts II. He first appears in a wire-frame form. This form is comprised of white lines forming a polygon mesh of triangles and quadrilaterals. It creates the silhouette of a human face stretched horizontally, with a large nose and wide mouth. The MCP's eyes are red pinpoints in this form.
The MCP's second form, his true form, is comprised of two massive, red cylinders that are being projected from a very small, spherical device between the two. This device is mostly silver with a red, circular light displayed on it. The light is surrounded by four black triangles. The bottom cylinder is featureless, while the upper one sports the MCP's face. Like in his wire-frame form, the MCP's face in this form resembles a human male's, albeit stretched across the cylinder. He has wide lips and a large nose, complete with nostrils, as well as faint "eyebrows". The MCP's sclera in this form are maroon, while his eyes are white. Only the first form is viewable in Jiminy's Journal.
At some unknown point, the system was copied by Ansem the Wise and used in his personal network. He created a dataspace for information concerning Hearts and the Heartless, and had Tron protect it. He also rebuilt the laser that allows Users to travel into the computer, and stored away the MCP, realizing the danger he posed to the system, and to Users themselves.
After the banishment of Ansem the Wise, Xehanort and the other apprentices reactivated the MCP, possibly to gain access to Ansem's private files.
Years later, the Hollow Bastion Restoration Committee rediscovered Ansem's computer, and booted it up, discovering the MCP still active. He had been using Ansem's and Xehanort's data to create an army of digital Heartless, put them under command of his number two, Sark, and locked away Tron. No other programs were ever shown, though were likely either destroyed and replaced by Heartless or out of sight, mindlessly working for the MCP.
The MCP himself attempted to gain access to the DTD, Ansem's dataspace, to acquire infinite knowledge and take over the world. It was for this very reason that he did not want to de-resolute Tron as per Sark's suggestion, because the MCP discovered that Tron had escaped, and had access to the dataspace. For a short time, the MCP did have access to it, and immediately activated the emergency self-destruct mechanism for the town. But with help from Sora and Co., Tron was able to keep him out, and created a new password.
The MCP, however, was able to fight from another front to take over the world. He reactivated the Heartless manufacturer in Ansem's lab, and began creating an army of electronic Heartless to ravage the streets of Hollow Bastion. Although Leon and Yuffie fought them off well enough, the Heartless kept coming, and their only hope was Tron, who was locked away in the Hollow Bastion OS. Cid began making an MCP Eradication program, enhanced with Merlin's magic. Sora then used it to save Tron in the digital world.
After Tron received the enhancement from Cid, he and Sora went to the Central Computer Mesa, and fought the MCP himself, as well as an enlarged Sark. The MCP tried to to convince Tron that they could work together without the help of Users, but Tron retaliated.
Tron was eventually able to use Cid's program to delete the MCP, and he quickly jumped into the Master Control Program's resting place, seemingly killing himself. He is soon shown to have taken over the MCP's job, however, and now runs the entire system according to the wishes of the Hollow Bastion Restoration Committee. He also gives them a brief glimpse of what the world used to look like, which led them to remembering the town's real name: Radiant Garden.
The MCP does very little fighting himself. Instead, he resurrects Sark as a monstrous figure to fight for him. Sark would have to be incapacitated for the attack on the actual MCP to begin. For this to be done, Sark's legs would have to be damaged enough for him to kneel over, and then his head would have to be attacked. A quicker way to disable Sark is to run away from him until he would put up a barrier in front of Sora. Here, Sora can use the Reaction Command "Climb" to quickly ascend the wall and use the Reaction command "Needle Dive", diving through Sark's head and knocking him out in one hit.
Once he was knocked out, Sora and company would have to destroy one or more of the panels shielding the MCP from view, and then upload the Eradication Program with Tron to the MCP with a button-mashing Reaction Command. Eventually, The MCP will revive Sark, starting the process all over.
The MCP also has a secondary defense mechanism. He can rotate the panels around him, and fire lasers from them, making it tricky to fight Sark and find a place to start deleting him.
The MCP, according to dialogue in the original film, started out as a mere chess program, but soon became the network controlling program of a company called ENCOM, where he was able to monitor and control the progress of functions of the entire company. As time wore on, he began to make himself more powerful by deleting weaker programs and adding their functions to himself.
Eventually he was too powerful to be stopped even by a User, and was only stopped from taking total control of every computer network by a lone User, Kevin Flynn, who was pushed into the digital realm, with the help of a powerful security program called Tron, written by Flynn's co-worker and close friend, Alan Bradley.