Rachel Walling is a Special Agent with the Federal Bureau of Investigation. Her father, Harvey Walling, was a cop in Baltimore. Her mother left when Rachel was young, so she was raised primarily by her father. He may have molested her but she refuses to talk about it. Her father died of gunshot in their home when she was a teenager. It was ruled a suicide but there were many suspicious or unexplained factors. Later, she attended Georgetown University where she majored in psychology. She initially wanted to become a diplomat, but after college she was recruited by the FBI and started at the New York field office. While there, she attended night classes at Columbia University to obtain a law degree.
One of her early projects was a study of suicides by police officers, prompted by her experience with her father. Another project involved interviewing imprisoned killers for the VICAP database, working alongside Robert Backus. She and Terry McCaleb investigated the murder of the Showitz family in the Florida Everglades, but never arrested the killer.
She was partnered for a time with fellow agent Gordon Thorson. They fell in love, married, and then divorced 15 months later. After their divorce they continued to work together, but tension between them led their management to look for a way to move one of them away from Quantico. She was briefly involved with journalist Jack McEvoy in 1995 when their paths crossed in pursuit of the serial killer called the Poet. The study of cop suicides became important because the Poet was killing detectives and making it look like suicide. Walling, Backus and McEvoy pursued the Poet to Los Angeles where they believed he was about to kill another cop, LAPD detective Ed Thomas. She ultimately realized that Backus was the Poet and, at an abandoned house in the Hollywood Hills, she shot him but he escaped.
The aftermath of the Poet case was messy and embarrassing for the FBI, and a scapegoat was needed. Although she had identified the Poet, she was disciplined for her unprofessional relationship with McEvoy, resulting in her being transferred to the FBI field office in Minot, North Dakota. In 1998 she was transferred again to Rapid City, South Dakota.
In 2004, she was called to the town of Zzyzx in California after the reappearance of Robert Backus. A GPS reader had been sent to her attention at Behavioral, with a waypoint labelled "Hello Rachel." Its coordinates were a location in the Mojave Desert near Zzyzx. She was invited to participate as an observer because of her past relationship with Backus. In the course of the investigation she became reacquainted with Harry Bosch who was looking into the death of Terry McCaleb. Walling and Bosch worked together to locate Backus's hideout in Nevada. Then, working on Bosch's hunch they located Backus in L.A. They were able to prevent a second attempt on the life of Ed Thomas, and Walling helped rescue Bosch from the rain-swollen L.A. River. She and Bosch had an on-again/off-again relationship during and after that case.
In September of 2006, she received a call from Bosch requesting a profile of the serial killer Raynard Waits. By that time she was in the Tactical Intelligence Unit of the L.A. Field Office. She explained the hidden meaning of Waits's name and helped Bosch uncover his true identity. She also told Bosch that several aspects of Waits's confession are inconsistent with her profile of him. While working together they renew the sexual relationship that they had several years before. She was with Bosch when they went to Waits's hideout in Echo Park, but she strongly disagreed with his decision to go in without backup. Later she was present when Abel Pratt was shot by Anthony Garland. She accused Bosch of knowing that Anthony would be armed and that he might try to kill Pratt. She let Bosch know that she could not stay in a relationship with him because of his tendency to put himself and others at risk by not following standard procedure.
In 2007 she was still in Tactical and partnered with Jack Brenner. Prior to that she had been partnered with Cliff Maxwell. She and Brenner met with Dr. Stanley Kent and his wife Alicia about potential dangers of his profession, medical radiology. Later, Walling was called to the site of Dr. Kent's murder, where she encountered Harry Bosch, and together they rescued Alicia Kent at her home. The case initially appeared to be terrorism-related with a group coercing Dr. Kent to provide them with radioactive cesium, then killing him. However, evidence acquired by Bosch eventually convinced her that the terrorism angle was misdirection and the real killers of Dr. Kent were his wife and Maxwell. Walling and Bosch pursued Maxwell to the building where Alicia was being held. They engaged in a shootout with Maxwell inside the building but no one was hit.
In May of 2009, she received a call from McEvoy regarding his identity being stolen while he was working on a story in Las Vegas. She took a federal jet from Los Angeles to Nellis Air Force Base in order to meet him at the Hotel Nevada. She was later forced to quit the FBI in order to avoid prosecution for misuse of government funds in her use of the jet, but she was reinstated after McEvoy shared evidence that linked Freddy Stone to the murders of Sharon Oglevy and Denise Babbit. Walling and McEvoy briefly rekindled their relationship, but McEvoy described her at that time as being like a feral cat, never permanently attaching to anyone.
In February 2010 Bosch contacted her to get an unofficial profile on Jason Jessup. She indicated that he was possibly a serial killer and that the stress of trial might push him to kill again. She followed the case after that but did not play an active role.
In November of 2014, she met with Bosch again to aid in his investigation into white supremacist and anti-government activist Rodney Burrows who had served time for tax evasion. Burrows was a suspect in the EZ Bank robbery in 1993, but Walling was unable to provide much information. At this time she was still involved with Jack McEvoy, making it more of a long-term relationship.
Rachel has a complex personality. She has problems with authority resulting in frequent clashes with her superiors in the FBI. She dislikes following orders, sometimes with negative consequences for her. She makes friends but does not stay in touch with them. When she learned that McCaleb had died, she refused to cry even though she felt guilty for not contacting him when she had a chance. Her sex life appears to be highly sporadic, mostly involving men that she should be avoiding for professional reasons. Although she is usually all business, she also has a playful side that comes out rarely. She once told McEvoy that she wanted to be called by the alias "Agent Misty Monroe" in his book about the case they were working on.