Prior to her transfer to Texas, Ruiz once worked as a policewoman for the Brooklyn North Homicide Unit in the New York City Police Department, where she was well-respected and liked. One of her cases at New York was the murder of a couple that was witnessed by the couple's daughter, who described the perpetrator as a "dragon." Never forgetting about it, she has since kept a newspaper article about the case. Moving to Terlingua, Texas, to semi-retire in October 2009, she took over as Sheriff for the local Sheriff's Department, where she met the deputies, including Ronald Boyd. Because of the high crime rate in the area, as well as the few deputies that could support her, Ruiz showed immediate displeasure at her new job, later citing that there was practically no one she could trust. She then spent the next six months tracking dozens of cases of missing illegal immigrants who were killed by Boyd. As a result, he became intimidated by her and sent the heads of three of his victims, one of which was cut off recently, over to the police station. This leads to her calling the BAU in to aid the ensuing investigation.
A Rite of Passage
Ruiz is present when the BAU interrogate Omar Morales, who is being suspected of the murders. However, he is eventually let off the hook. She and the BAU later realize that the unsub is making it personal for her when he discards the head of his latest victim at her home. When the BAU interrogate the immigrants that were traveling with the latest victim, one of them, a child, mentions seeing "The Sun" when it was nighttime. The following night, Boyd approaches her outside and tells that he believes the child might be referring to an abandoned Gold Harvest barn, whose symbol is the Sun. The two of them drive to the barn, where they spot the barn's other symbol, Santa Muerte, before Ruiz realizes that Boyd has a bad leg and remembers the profile given by the BAU, which suggests the unsub is handicapped in some way. After having a brief talk with Boyd, he states his intention to kill her and that she won't make it. Though she had had little-to-no chance of outgunning Boyd, she pulls out her gun and tries to shoot him, but Boyd is faster and instantly kills her. Afterward, Boyd guts her, cuts off one of her hands, and cuts out her eyes and tongue in hopes of framing Omar Morales, who, during his earlier interrogation, mentioned he would do the aforementioned mutilation process to whomever he murdered. Unfortunately for Boyd, the plan fails and when he is tracked down by the BAU, he is shot dead by Morgan when he retaliates violently with an assault rifle. The BAU later make arrangements for her body to be sent back to New York City, where she will have a proper burial.