"I know Aaron's worried. Tell him I'll protect them [Haley and Jack] like they're my own."
Sam Kassmeyer was a U.S. Marshal who appeared in Season Five of Criminal Minds.
Background
Not much information is specified about Sam's early life, other than he was somehow acquainted with Hotch and had become a U.S. Marshal.
Season Five
Nameless, Faceless
Sam is assigned to protect Hotch's wife Haley and their son Jack, putting them into protective custody after a serial killer named George Foyet attacks Hotch and makes his intention to kill Haley and Jack known. He reassures the BAU that Haley and Jack will be safe under his watch before leaving with them.
Reckoner
Sam reappears in Reckoner, showing Hotch a video of Jack on a swing. He tells him that he will continue to keep Haley and Jack safe while the BAU continues to track down Foyet.
100
In 100, Sam is tracked down by Foyet, who shoots both of his legs, one foot, and severs several of his fingers in an attempt to get Haley's location. Sam remains silent, not revealing any information. Foyet then beats him severely before leaving, and Sam would later die en route to the hospital. Before his death, he tells Hotch that he did not reveal anything about Haley and Jack's location, but finds out anyway through the speed dials on his cellphone.