|“||I know Aaron's worried. Tell him I'll protect them like they're my own.||”|
— Kassmeyer in "Nameless, Faceless"
Kassmeyer 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.
Kassmeyer reappears 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.
In his final appearance, Kassmeyer is tracked down by Foyet, who shoots both of his legs, one foot with an M1911, and severs several of his fingers with a knife in an attempt to get Haley's location. Kassmeyer remains silent, not revealing any information. Foyet then beats him severely before leaving, and Kassmeyer 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 cell phone.