Detective Mark Reyes is an incidental character who appeared in The Return.
Absolutely nothing is known about Reyes's personal life prior to him becoming a police officer. In 2006, he stumbled across the illegal activities of fellow officers Wayne Gulino and George Scotman, both of whom were brutally assaulting suspects in order to extract confessions out of them. He filed a report, resulting in an Internal Affairs investigation and then the stripping of Gulino and Scotman's badges. This inadvertently earned him Gulino's wrath, as he felt defined by his badge and that he was wronged. Reyes moved on in the past seven years, eventually rising to the rank of Detective.
When a teenage boy named Gavin Rossler, who had been missing for four years, opened fire at a diner, killing three people before being shot dead by a plain-clothed police officer, he calls in the BAU to assist the investigation when it is discovered that someone dropped Gavin off at the diner. Later, the driver, identified as another missing boy named Trevor Madison, commits a suicide bombing at an intersection, killing a traffic cop. He and Morgan later arrest a third missing boy, named Daniel Morrison, after he attempted to open fire at a park. During the arrest, he is briefly stunned after Daniel delivers him a hateful expression on his face. Later, Garcia identifies Gulino as the one who abducted and influenced the boys, and that Reyes was the one who stripped him of his badge.
Blake attempts to interrogate him about Gulino when Stephanie Lawford, another abduction victim of Gulino's, enters the police station, wearing a vest rigged with explosives. She demands for Reyes, to which he promptly faces her in an attempt to negotiate with her. However, Stephanie delivers him a message from Gulino and tries to detonate the suicide vest, but is shot and injured by Morgan. The BAU then deduce that Gulino is headed towards the last known residence of his partner Scotman (who died in 2008 of cancer); there, they confront and arrest him. Reyes is last seen taking Gulino into a police cruiser, reading him his Miranda rights while doing so.