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 is not mentioned whether or not Yates' father was ever 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).
Two new murders in Miami feature a unique trait - each of the victims had their skulls sawed into and a portion of their brain removed. The killer is dubbed The Brain Surgeon (courtesy of Joey Quinn).
Dr. Evelyn Vogel offers to assist Miami Metro with the investigation. Soon she approaches Dexter Morgan seeking his help with a related matter. She believes that she is in danger from this very 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 enlisting Dexter, they cross off various patients from the list (including a cannibal by the name of Ron Galuzzo) and arrive at "Chapter 7" (in reference to her novelized account of psychopaths, through her own records), A.J. Yates.
Dexter investigates Yates, locating him outside of a home where he is installing cable for his job with Solray Communications. While reading through his history, a particular trait catches Dexter's eye - a scar located on the back of Yates's head that closely resembles that of the victims to The Brain Surgeon. Dexter later questions Vogel, learning that she suggested in his youth that a hospital do a surgery on him to remove a lesion on his brain (that she suspected created his violent tendencies). The two begin to suspect the surgery as motivation for him potentially targeting Vogel.
Later, arrives at Yates' house. He doesn't see Yates's van and assumes he's at work. He heads inside, investigating each room before reaching the bedroom and discovering 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 others throughout the house) where 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 stalks towards Dexter but stops when he Dexter calls someone and he hears his former psychiatrists' voice on the other end. After Dexter finishes his conversation with Vogel, Yates returns to his lair and says to himself, "Vogel's found herself a hero." He sits back at the monitors watching Dexter. A woman is shown chained to the wall 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 no one there. However, he discovers the entrance to the lair and makes his way down into it, locating the monitoring equipment (realizing Yates knew he was there the entire time). He also discovers manuals, as well as 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 was previously chained to a wall). Although Janet was stabbed, there is pressure on the wound due to her enclosed space which is keeping her alive. Dexter rescues her (still unconscious) and she later gives a statement to the police.
Afterward, Dexter returns to Yates' vacant house with 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 on himself, written by Vogel. Dexter is furious with Vogel 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 hurry to his side. Yates arrives at the hospital, rushing 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 the oxygen tube on his father and using the bedside alarm as a distraction against Dexter. As he escapes through a window, hospital staff rush into the room. Dexter quickly hides behind the door and slips out during the commotion, but learns 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 on who the guy was going through his house. Vogel doesn't thoroughly explain herself, feeling that Yates is TBS (The Brain Surgeon) but states that Dexter is a friend. She tells him that the surgery he had as a child is why he is with her. 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 again pulls a bag over her head.
Meanwhile, an investigation is underway for Yates after his surviving victim Janet Thorton gives up a positive ID on him. They arrive at his house, where the bodies of three women are unearthed in the yard, with their graves marked by rose bushes. The remaining bushes indicate that there could be 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 learns what he can from Miami Metro Homicide's investigation, his sister, Debra Morgan, arrives, after she discovers Vogel's house ransacked and she has gone missing. 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. They converse about his past, with Vogel analyzing him and determining his actions are based on his abusive mother during his childhood. Yates disapproves of talking about her but does begin letting loose some of what she did to him, while Vogel continues to coach him in hopes of winning him over. It's not long before he becomes frustrated and instead tells her to shut up, moving down to her foot, ready to break each of her toes slowly (as per his ritual). Vogel, figuring the only way to control him at this point would be to take drastic measures, smacks him across the face once. Noticing how he stopped and seemed to be effected emotionally by the attack, she strikes once more...and attempts a third time before he stops her. After forcing her to sit, he moves into the kitchen to clean up a cut lip. Meanwhile, Vogel quickly reaches for a cellphone to call Dexter. She is able to make the call but has to leave the phone face down before Yates returns, thus allowing Dexter to listen to their conversation (and hopefully provide a way to pinpoint their location).
Dexter and Debra listen in on the call, with her using her current boss, Jacob Elway, to trace the call remotely. After determining the address, the two begin driving to the location. In the meantime, Yates can be seen sitting on the couch claiming that he can't be helped, but Vogel reassures him that she's there to aide him. She talks to him, breaking down his defenses as he seems in tune with her understanding words. Suddenly, Yates realizes that a phone is on nearby and he juts away from her as Vogel 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 bloody rags in the kitchen (which Yates used to clean his injury). When they hear a noise upstairs. they run up to the master bedroom and check the closet where Vogel is found gagged and 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 where he locates a long curtain rod shaped like a spear. Yates, who is under the bed, has a knife ready as Deb slowly walks backwards towards it. 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.
Under a unique set of circumstances, both Debra and Vogel are present on the Slice of Life as Dexter dumps Yates into the ocean. In tune with more recent victims, the body is not cut up and wrapped entirely in black plastic.
The Morgans and Vogel continue their lives feeling that The Brain Surgeon is finally gone from their lives with the death of Yates, however unfortunately for them and Dexter's recent protege Zach Hamilton, the real Brain Surgeon makes his return in "Are We There Yet?" after killing Zach and leaving his body in Dexter's apartment. This suggests that the real TBS set Yates up (by leaving the instructional manuals, jars and patient files in his house) as a way to mislead both Dexter and Vogel after he had already thrown off the police by framing Lyle Sussman.
- Kendall Jaynes (first victim, missing since 2008)
- Three unnamed women
- Kathleen 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 portrays A.J. Yates in Dexter, has previously co-starred in the UK version of Shameless.