Montrell Perez was the leader of a gang of Navy personnel who planned to pull off a five million dollar electronic heist, resulting in NCIS Special Agent G. Callen going undercover as a dirty M.P, "Jason Tedrow" to nail the people behind it.
Once the operation was completed, all those involved were arrested and as a result of Joey Gale and Jon Donnelly testifying against him, Perez got ten years in prison, leaving him bitter.
Nate Getz described Perez as a "sociopath with gang ties" with connections to the Fourth Street Flats.
Perez even developed an interest in Agent Kensi Blye much to her disgust but as he was leaving the main room after Kensi had interviewed him for a second time, Perez collapsed.
Despite Kensi's efforts, Perez died almost immediately and Dominic Vail's findings later revealed that Perez had been murdered after someone had poisoned him with sodium cyanide.