Sark is incharge of the whole Game Domain and is the MCPs
right-hand man. He travels around the domain in his Command Carrier.
Sark is assigned by the MCP to destroy Flynn on the Game Grid. MCP basically tells Sark, "Train him for the games, let him hope for a while, and blow him away!" Sark takes his job seriously.
If you haven't watched the film you've gotta get the 2 disc 20th anniversary Tron DVD. It's worth it.