A former patient of Dr. Evelyn Vogel, he worked as an electronics technician specializing in cable and internet installation. Behind the mask of a technician, he was a psychopath with a fixation on feet and killing women.
He was initially believed by Dexter Morgan and Evelyn Vogel to be The Brain Surgeon serial killer after discovering brain surgery diagrams in his house, and Vogel's patient files on his computer. However, Yates was later confirmed not to be TBS  because the real Brain Surgeon killed another victim.
Yates is a man in his mid-30s. He keeps his black hair shaved short around the sides and back of his head while, on the top, his hair is longer and slightly spiked.
A unique feature to Yates is that he has a large scar around the back of his head where he received brain surgery in his youth to remove a lesion that was thought to cause his violent behavior.
His work uniform consists of a pair of brown khaki pants with a matching shirt as well as a black cap with his company's name on it.
Yates' regular life is never thoroughly observed, but he is a very cautious individual as his entire household is wired with security cameras and he has a set of monitors hidden away in his basement (lair). Also, he is a very neat individual, with his house kept in order.
He is cunning and intelligent, being able to keep women in captivity for weeks at a time in his house (which is located in a suburb, not a secluded area).
Yates appears to care about his father who resides in a retirement home. However, Yates didn't hesitate to risk his father's life in order to save his own when Dexter lured him into a trap.
He has a strong dislike for his own birth name "Albert," which his mother used during his childhood while abusing him. This weakness is exploited by Vogel when she is held captive by A.J. After she calls him "Albert" and slaps him, he temporarily reverts to a terrified state. Despite being physically hit by Vogel several times, he does not harm her. He takes some time to think, and then sits on the couch with Vogel to talk about his issues. When Dexter Morgan arrives to rescue Vogel, Yates hides under a bed, just like in his youth.
Yates grew up with an abusive mother. When enraged, she often beat him and stalked him. He would hide underneath his bed, feeling that it was his only safe place. As she called his name, he watched her feet pace back and forth. It's not mentioned whether or not his father was abusive to him.
At the age of twelve, he attacked a classmate with a chair and, at age fifteen, he was institutionalized. Dr. Evelyn Vogel worked with the boy, trying to channel his violent tendencies, but he was beyond her help. Eventually, hospital staff conducted an X-ray of his brain, identifying a lesion that Vogel suspected might contribute to his violent behavior. She suggested surgery to remove it. The hospital went through with the operation, unknown to Vogel herself, leaving Yates with a scar from the procedure.
Later in life, Yates began to capture women and keep them in captivity for days. He had an unusual fixation with their feet, breaking each individual toe on the right foot, while leaving the left foot untouched. Vogel believed that he targeted women due to his anger toward his mother, seeking an obscure connection. After the women died, he would bury their bodies in his backyard with all of their possessions, minus the right shoe, which he kept on display as a trophy in his closet.
Dr. Evelyn Vogel offers to assist Miami Metro with the investigation. Soon she seeks Dexter Morgan's help with a related matter. She believes that she is in danger from the same killer. The missing brain portions had been placed in jars and left as "presents" for Vogel on her doorstep. She feels threatened and suspects that the killer may be one of her former patients
After Vogel enlists Dexter, he vets and begins crossing off the patients she referenced in her book.
Dexter next investigates A.J. Yates, spying on him outside of a home where he is installing cable for Solray Communications. A particular trait catches Dexter's eye. A scar on the back of Yates's head closely resembles the cuts on the Brain Surgeon's victims. Dexter later questions Vogel, learning that she suggested that a hospital do a surgery on him to remove a brain lesion which she suspected created his violent tendencies. The two think the surgery may be motivation for him to target Vogel.
Later, Dexter arrives at Yates' house to find evidence. He doesn't see Yates's van and assumes he's at work. He heads inside, investigating each room before reaching the bedroom where he discovers an odd shrine dedicated to six right female shoes. Unknown to Dexter, there's a security camera located behind these shoes (and at least seven other cameras throughout the house) and Yates has been watching him since he entered. Yates calmly turns around in his chair, stands up and grabs a hand taser before exiting his lair through a secret entrance in the kitchen. He moves stealthily toward Dexter, but stops when Dexter makes a phone call and he hears his former psychiatrist on the other end. Yates returns to his lair and says to himself, "Vogel's found herself a hero." He sits back at the monitors, watching Dexter leave. A woman is shown chained up behind him, alive.
Dexter learns that the fingerprints he lifted from the shoes belong to missing women, and this confirms that Yates meets the Code of Harry. He heads back to Yates's house, where he intends to capture Yates but finds him missing, along with the shoes. However, he discovers the entrance to the lair and makes his way down into it. He finds the monitoring equipment and realizes Yates knew he was there the entire time. He also discovers manuals, as well as specimen jars and equipment for brain surgery. This convinces Dexter that Yates is the Brain Surgeon. Before he departs, he hears a noise in one of the toolboxes, and opens it to discover Yates' most recent victim, Janet Thorton, who had been chained to a wall. Although Janet was stabbed, there is pressure on the wound due to the enclosed space which is keeping her alive. Dexter rescues the unconscious woman and drops outside of an E.R. She later gives a statement to the police.
After dropping Janet at the E.R., Dexter returns to Yates' vacant house with Dr. Vogel, and the two of them agree that Yates is the Brain Surgeon. They have a falling out when Dexter discovers that A.J.s' computer has files about him, written by Vogel. Dexter is furious and states that after he deals with Yates, she will be out of his life
Dexter arranges a way to trick Yates into appearing before him by using his sickly father, whom he learned about during his time in Yates' house. Dexter calls Yates, pretending to be a hospital attendant saying that his father's heart is failing and he should come quickly. Yates rushes to his father's side, saying that he's there for him. Suddenly, Dexter closes the door behind them, announcing his presence. After a short exchange, Yates pulls a surprise move by removing his father's the oxygen tube to use the bedside alarm as a distraction. Yates escapes through a window as hospital staff rush into the room. Dexter quickly hides behind the door and slips out during the commotion, but he's learned that Yates is capable of anything.
Vogel is in her house listening to a song while reading through her mail. Suddenly, a chair is thrown through her window and Yates crashes in, tackling her and threatening to kill her if she tries to escape. He kidnaps her and brings her to one of the vacant houses for his work, where he questions her about the guy who broke into his house. Vogel says little, feeling that Yates is the Brain Surgeon, but states that Dexter is a friend. She explains that the surgery he had as a child was intended as way to reduce his violent tendencies. He laughs that off, mocking the scar before moving to the other side of the room. He accuses her of interfering in his life (while she believes he had stalked her). After they talk a few more minutes, he pulls a bag over her head.
Meanwhile, an investigation is underway for Yates after his surviving victim, Janet Thorton, provides a positive ID. At Yates' house, the bodies of three women are unearthed in the yard, with their graves marked by rose bushes. The remaining bushes indicate that there is likely at least three more victims. Each of the victims had her toes broken separately over time, then stabbed to death, and buried with her purse, left shoe, and I.D. While Dexter is learning what he can from Miami Metro Homicide's investigation, Debra arrives, having discovered Vogel missing and her house ransacked. The two begin searching all possible locations Yates that could have taken Vogel, figuring it to be one of the vacant houses where he's assigned.
As their search continues, things intensify between Yates and Vogel. She tells Yates that his actions are based on his abusive mother during his childhood. Yates disapproves of talking about his mother, but opens up about what she did to him, while Vogel counsels him in hopes of winning him over. It's not long before he becomes frustrated and tells her to shut up, moving down to her foot to slowly break each of her toes. Vogel decides that the only way to control him at this point is to take drastic measures, and smacks him across the face once. Noticing how he stopped and seemed emotionally effected by the attack, she strikes again... and attempts a third time before he stops her. After forcing her to sit, he walks into the kitchen to clean up a cut lip. Meanwhile, Vogel finds the door locked and quickly reaches for a cell phone to call Dexter. She is able to make the call but must leave the phone face down before Yates returns. This allows Dexter to listen to their conversation and hopefully provide a way to pinpoint their location.
Dexter and Debra listen in on Vogel and Yates, while her boss, Jacob Elway, traces the call remotely. After determining the address, the two drive to the location. In the meantime, Yates is sitting on the couch, claiming that he can't be helped,. Vogel reassures him that she's there to aid him. She begins to break down his defenses until he suddenly realizes that a phone is on nearby. Yates angrily juts away from her as she calls out to Dexter for help.
Dexter and Debra soon arrive at the house and bust open the door, only to find no one in the living room. They search the downstairs, finding the bloody paper towels in the kitchen that Yates used to clean his injured lip. When they hear a noise upstairs, they run up and find Vogel in a bedroom closet, gagged with her wrists tied by duct tape. She isn't certain where Yates went, but Dexter notices a blood stain next to the bed. He looks around for a weapon, casually playing off that they "need to look for Yates." He sneaks to the window and takes a long curtain rod shaped like a spear. Yates, who is under the bed, has a knife ready as he watches Deb's feet. Just before he can slash at her leg, Dexter jumps on the bed and thrusts the pole straight down, impaling Yates who dies within seconds.
The Morgans and Dr. Vogel believe that The Brain Surgeon is finally gone from their lives with the death of Yates, Unfortunately, the real Brain Surgeon makes a return in "Are We There Yet?" This implies that the Brain Surgeon left the instructional manuals, jars, and patient files in Yates' house as a way to mislead Dexter and Dr. Vogel, as he did with Lyle Sussman.
- Kendall Jaynes (first victim, missing since 2008)
- Three unnamed women
- Kathryn Pickett
- Isabella Scannone
- A calendar in A.J. Yates' house shows that his father's birthday is 9 July 2012.
- There are some similarities between Yates and another psychopath named Boyd Fowler, appearing in Season Five. The characters wear similar uniforms and both kidnapped and tortured women. While Boyd kept trophies of women's hair, Yates kept their shoes.
- Aaron McCusker, who plays A.J. Yates in Dexter, has previously co-starred in the UK version of Shameless.