Born sometime in 1970, Armando was a Mexican national who was forced to sleep on the floor as a child due to his family's poverty, leaving him with a deep desire for his own bed and home, which he confided in his half-brother and colleague Ruben Garcia. He was arrested and imprisoned for an unknown amount of time for various cases of burglary and assault. After being released, Armando moved to California with Ruben to work on the farms, and the two joined several others to form a "cuadrilla", a group of migrant workers. On August 25, 2008, due to his drug habits, inability to get along with others, and also a general dislike of work, Armando was kicked out of the group by Ruben after costing them a big job in Tehachapi, causing him to snap. Two days later, he found the home of Mildred Younce, an elderly woman who gave food to hobos in exchange for doing odd jobs. Killing her, Armando then spent the night in her house. Leaving the next day, Armando began to hop from boxcar to boxcar to follow his half-brother, killing homeowners, living in their houses, and selling any valuables that he took from them, leaving the money in farm camps for Ruben to find.
By September 28, 2008, Armando begins killing more frequently, having murdered the Sullivan and Ortiz couples in the span of a single day; he finds a local newspaper mentioning him in the latter's home. Amused by the paper dubbing him the "Highway 99 Killer", the authorities having incorrectly guessed that he moves along the highway. Armando places the paper between the dead husband and wife, under the bowling trophy he killed them with, as a taunt. Hopping a train, Armando reaches another work camp and leaves some more money for Ruben. Later brought to the town of Lockeford by another train, Armando finds a suitable house, grabs a shovel lying in the garden, and prepares to break inside. However, the resident Rose, who had been asleep on the couch, wakes up and hears a news broadcast about Armando just before spotting him outside through a window. Fleeing to a nearby rail yard, Armando boards a boxcar and, when a railroad police officer climbs in to look for him, Armando kills him with a rail spike. Spotted by Morgan while murdering the guard, Armando climbs on top of a moving train, followed closely by Morgan. While Morgan is climbing up after him, Armando starts trying to kick him off the train. Right before he can send Morgan plummeting to the ground, Armando is killed when Hotch shoots him in the chest from a moving car. Prentiss then tells the news of Armando's death to Ruben, which devastates him.
The unsub is a Hispanic male transient aged somewhere between twenty-five and forty-five, who would be physically fit but somewhat battered from train-hopping and fighting with other homeless men. He may look homeless, but he is taking clothing from his victims' homes, so he will be the only transient near the railroad tracks wearing clean clothing. The trains and the rail-yards are his real home, and when he gets tired of them, he chooses a house to make his own. He will have a red dry rash around his nose from sniffing chemicals, which is what is commonly known as a "sniffer's rash". He takes household cleaners (such as nail polish, glue, paint thinner, and lighter fluid) and abuses them as psycho-active inhalants. It is believed that he is fulfilling some sort of fantasy inside the victims' homes. Upon leaving, he takes his victims' clothing, money, jewelry, and small electronics. If a person gets close to him, he or she will notice him and describe him as smelling "like a combination of human filth and paint thinner".
He shows characteristics of both an organized and disorganized killer; he brutally beats his victims to death with a random object that he finds lying around, but follows a "ritual" afterward by wiping away his fingerprints and symbolically dressing his victims in his own clothes by laying them on the bodies. By destroying others and then living out their lives, he is raising his own self-worth.
Following his half-brother by riding the rails, and after leaving the boxcar that he had stowed away in, Armando would search for homes within one mile of the tracks, breaking into one at night which lacked security features such as dogs, alarms, and motion sensitive lights. Once inside the house, Armando would bludgeon the inhabitants to death while they were sleeping in their beds, using any heavy object in the home that caught his eye. With the homeowners dead, Armando would wear their clothes, put his clothes on top of his male victims' bodies (and presumably his female victims if they lived alone), use their facilities and commodities, take various small valuables, and get high off of any household chemicals that he could find. Armando would send any money that he made by pawning items that he stole to his half-brother, leaving the cash in an envelope on the notice boards of worker camps. He would also wipe anything that had his fingerprints off in the homes as a forensic-countermeasure before leaving, except for Mildred Younce's crime scene, since it was his first murder and he hadn't fully developed his style yet. As his fantasy and desire to live in a home increased, his cooling-off period accelerated and he started killing daily. When he killed the railroad police officer, he stabbed him repeatedly with a rail spike.
Armando appears to have been inspired by Juan Corona - Both were Mexican-American serial killers who were active in California, worked on farms, and had half-brothers who also immigrated to California.
Armando may have also been inspired by the unidentified Original Night Stalker - Both were serial killers and cop killers who attacked couples in their homes, killed them by bludgeoning them to death with an object from the house, and were given nicknames for their crimes.
Armando was based off on serial killer Ángel Maturino Reséndiz - Both were serial killers and robbers who were Mexican drifters, traveled along railroads all over the U.S., killing victims everywhere they went (and mainly by bludgeoning them), both had jobs as migrant farmers at some point, and were given nicknames for their crimes.
Armando is also partially similar to K.P. Jayanandan - Both were serial killers who broke into their victims' homes, killed their victims by bludgeoning them and robbed them afterward, killed members of law enforcement who were also their last victims, and both were given nicknames for their crimes.
- Unspecified dates: Numerous unspecified vicimless burglaries and assaults
- 2008, California:
- August 27, Tehachapi: Mildred Younce
- September 6, Bakersfield: Unnamed victim
- September 9, Orange Cove: Sue Smauder
- September 13, Fresno: Barry and Gina Jones
- September 19, Vacaville: Catherine Hexly
- September 21, Chico: Jeff Davidson
- September 25, Sacramento: Alan and Brenda Paisley
- September 27, Modesto: Larry and Ann Sullivan (bludgeoned with a clothing iron)
- September 28:
- Stockton: Mr. and Mrs. Ortiz (first names unrevealed; bludgeoned with a trophy)
- Rose (intended to bludgeon with a shovel, but fled when she woke up)
- Unnamed railroad police officer (incidental; stabbed at least six times with a rail spike)
- Derek Morgan (attempted; tried to knock him onto the railroad tracks to get him run over by a train)
- Armando Ruis Salinas is the fifth of only eleven serial killers in the show's history to have killed at least a dozen victims, yet not be a prolific killer because the span of their crimes was less than three years. The others are:
- Season One
- Season Two
- Season Three
- Season Four
- Season Six
- Season Seven
- Season Eight
- Season Thirteen
- According to his file from the Department of Corrections, Armando's Social Security number is 960-28-1959.