McSweeten is a Special Agent with the Federal Bureau of Investigation (FBI). His partner is Special Agent Taggert.
When the Leverage team first met Special Agent McSweeten, he is working on the Nicky Moscone case in Los Angeles, California (The Wedding Job). Some time later, both McSweeten and his partner Taggert showed up in the desert town of Juan (The Bank Shot Job), where they arrested local meth dealers during a bank robbery.
Later, Special Agent McSweeten is responsible for transporting Mark Vector. However, he does link up with Special Agent Hagen (Parker) during the job, and explains that Taggert is on a special detail at FBI headquarters in Quantico, VA. He also partners with a Special Agent known only as Bob in The Boiler Room Job while Taggert is recovering from a ferret bite.
Every time they come in contact with Alec Hardison and Parker, both McSweeten and Taggert think that they are real FBI agents. And from McSweeten's very first encounter with them, it's quite obvious that he has a fondness for Parker.