In his youth, Ruiz grew up in foster care and did fifteen years in Apalachee Correctional Facility for trying to kill a police officer. He was also a member of Los Machetes, a violent Miami street gang appropriately named for their tendency to cut off the arms and heads of some of their victims with machetes. After being released in 2003, he apparently changed his ways, never committing a crime ever since. Sometime afterward, he got a job at a local soup kitchen and also became a religious practitioner. He eventually gained himself a small following for the latter services; one of his followers was a teenager named Elian Morales, whom he came to care for, helping him with his drug addiction. On December 19, 2010, he complained against Morales for assault, which resulted in the latter's imprisonment in juvenile hall, but eventually dropped the charges. Among the people Ruiz treated were the future victims of serial killer Hollis Walker.
Ruiz is first seen working at the soup kitchen, where Hollis's victims frequented, watching Reid and Morgan interview Jimmy Mercado, another frequent visitor. As Mercado leaves, Morgan notices Ruiz watching them and tries asking him some questions, but Ruiz replies that he doesn't speak to cops unless they have a warrant. Later on, the BAU track down Ruiz as a possible suspect in the investigation due to his background. When Reid and Morgan confront Ruiz during a religious ceremony at the local community center, Ruiz only agrees to come with them for interrogation after spotting Reid. This causes the agent to decide to interrogate him first at the police station. During the interrogation, Ruiz deduces that Reid is suffering from headaches, which are provoked by bright light. Morgan then enters the interrogation room and shows him photos of the victims killed by Los Machetes, angrily asking if he helped or just watched. However, Ruiz ignores Morgan and continues staring at Reid, eventually beginning to chant in African and tap his hand on the table, his voice growing louder and the tapping becoming more aggressive by the minute. Reid later described the occurrence as transpossession, a religious hysteria in which sufferers believed they were actually possessed by deities.
Minutes after the episode, Reid returns to the interrogation room, and Ruiz tells him that his saint spoke to him during the transpossession. When Reid recounts the words spoken to him, Ruiz translates for him, saying there was "something bad" in his head and that the saint was trying to clear it. Then, he says that he senses something is wrong and someone else is about to get hurt. Morgan then reenters the room and tells Ruiz that several items linking Elian Morales to the killings was found in his room, as well as a stash of heroin. This new information enrages Ruiz, who demands to look for Morales and cites that the boy must've fled from the BAU because he was simply using again. At this point, it becomes clear to the team that Ruiz is not the unsub due to his behavior, which didn't match that of a psychopath. Ruiz then begins a ritual to find Elian and the real unsub. He is then shown the crime scene photos, and he points out some things don't fit in with the religions the unsub is using as a basis for his crimes; the head wouldn't have been left behind by a practitioner who knew what he was doing and the tongue in the dish has no significance whatsoever, which leads the BAU to suspect Hollis of the murders. Ruiz is then released so he could find Morales, but not before he tells Reid that there are ghosts in his head and that they will eat his soul.
He searches around the neighborhood, asking anyone about Morales's whereabouts. Eventually, he goes to the soup kitchen and tries calling out to him, sensing his presence. When he enters another room, Ruiz is ambushed from behind by Hollis, who knocks him out with a pipe. Hollis then takes him and Morales (whom he also subdued) to an abandoned house, where he intended to kill both of them and frame Morales for all of the crimes he committed; the house was Ruiz's old foster home. As the BAU search for Ruiz and Morales, Reid deduces that Hollis is at the abandoned home and goes inside alone. There, Reid finds the both of them before being held at gunpoint by Hollis. At the same time, Ruiz regains consciousness and begins shouting at Hollis, but the duct tape on his mouth muffles out his words, rendering his voice to simple groaning. Hollis rants to Reid about his motivations, occasionally yelling at Ruiz to shut up. Reid then suffers from a headache and complains about the lighting of the house (although the lights were dim), which agitates Hollis. Then, he yells at Ruiz to shut up again, giving Reid enough time to subdue him with a pipe. As Ruiz is freed from his restraints, he asks if Reid came because he "heard [him] calling". However, Reid replies that he knew where they were because he saw a photo of Ruiz in front of the home. However, he doesn't believe it and gives the agent a small yellow bracelet, saying that it will protect him. Then, he leaves.