Like her son, Diana is a genius, although her IQ is never mentioned. She raised Spencer as a single mother after her husband William left her due to her schizophrenia (among other things). She read to Spencer often, enforcing his already growing intelligence. From her, Spencer learned that the best way to learn is to do "the research" on your own. Through the years, the reading time began to take place in her bedroom, where she spent most of her time on medication. When Spencer was eighteen, he made the difficult decision to finally have her committed. She resided in the Bennington Sanitarium, where she receives a letter from her son every day.
The Fisher King
Diana is first seen sitting in the sanitarium as Spencer watches. However, before he can come up to her, he is forced to get back to the BAU to assist on the latest case. She isn't seen for the rest of the episode.
When Spencer realizes that the unsub they are tracking down was once institutionalized in the sanitarium Diana is currently residing in, even during her stay, he fears that she might be the next victim and gets two agents to retrieve her from the sanitarium. Unfortunately, she is taken to Quantico via plane, in spite of her fear of flying. At the BAU headquarters, she and Spencer engage in their first conversation in years, during which Diana briefly expresses her distaste at not having seen her son in such a long period of time. When Spencer shows her a videotape of the unsub speaking, Diana instantly recognizes the voice and identifies him as Randall Garner.
When the BAU attempt to locate Randall's current address, Diana shows the team a photo of a medieval-styled mansion that was sent to her, with the address of said mansion written at the back. After Randall blows himself up upon being confronted by Spencer and a hostage of his is rescued, Spencer arrives back at the BAU headquarters to find Diana in the middle of a hallucination, believing that she is teaching a lecture in her class. Spencer goes along with his mother, asking if he can be a part of the lecture. When Diana asks if he read the material, he replies that it was read to him, to which she smiles and replies that it is the best way to learn it. Spencer and Diana are then both seen on the BAU's private plane as it heads back to Las Vegas, with Spencer comforting a fearful Diana by reading a book to her.
The Instincts (Part 1)
While investigating a case in Las Vegas, Spencer has a dream in which he is among those attending the funeral of the recently-murdered Riley Jenkins. Diana and a young Spencer appear, the former encouraging him to focus, causing him to wake up. While investigating Riley's murder, Spencer decides to interrogate Diana, but when he asks her of Riley, she dismisses the fact of him knowing Riley, stating that Riley was a figment of Spencer's childhood imagination. However, Spencer continues the investigation after having a dream depicting William discarding Riley's corpse.
Memoriam (Part 2)
After finding out the identity of Riley's murderer, Gary Michaels, Diana and William both come forward to him and tell him of how Diana became suspicious of Gary when he began stalking Spencer as a child. Diana alerted Riley's father Lou, who was distraught at the loss of his son, and after he discovered Gary's status as a pedophile, he and Diana arrived at Gary's residence. Diana watched Lou enter the house with a baseball bat at hand, and after a while, she too entered the house to find Lou standing over a dead Gary. Managing to get some of Gary's blood on her clothes, Diana and Lou both fled the scene and the former informed William. Fearing that Diana might be charged of being Lou's accomplice, he disposed her bloody clothes and Gary's murder was ultimately left unsolved, but the both of them were emotionally scarred by this and this resulted in their separation. Lou is presumably arrested afterwards for killing Gary, while Diana and William continue on separately with their lives, the resultant pressure from keeping their secret having been lifted.
While she didn't physically appear in the episode, it seemed that Diana's mental condition was beginning to improve, as she wasn't signed into the sanitarium as frequently and was actually allowed to have a supervised vacation to the Grand Canyon during the BAU's current case. By the end of the episode, Diana sent Spencer a number of postcards and a collectible miniature of the Grand Canyon.
Like in Season Nine, Diana doesn't physically appear in the episode, but she is the topic of a subplot. Spencer revealed to JJ that Diana's mental condition had begun to rapidly deteriorate, and sanitarium employees had to restrain her at some points. This worried him to the point where he began considering dilaudid again. JJ tells him to call her, and he says he will once the team finishes the current case. By the end of the episode, Spencer does indeed call her, and he asks if it would be okay for him to visit her for a few days.
While being held hostage by Cat Adams, Reid is forced to recount why he took a leave of absence to visit his mother. Reid then proceeds to tell Cat how Diana has schizophrenia and the new medication the doctors have prescribed recently hadn't been working. However, Reid also confesses to Cat how, when he visited Diana, she didn't immediately recognize him. Reid had Diana tested and the doctors diagnosed her as having early-onset dementia and would most likely progress into having Alzheimer's disease. A younger Diana then appeared in a flashback, pushing Reid on the swing of a park.
Spencer reveals to Rossi that Diana was accepted as a participant in a groundbreaking study on Alzheimer's, titled Metabolic Enhancement for Neurodegeneration, at Johns Hopkins University in Baltimore, Maryland. Spencer is excited by the news, as earlier tests had found ways to not only slow the disease but actually reverse it. Later, during a case, Spencer learned that Diana was found wandering around a casino, confused and not knowing who she was. She was safely taken in by police and has since recovered, but was embarrassed by the situation. By the end of the episode, it was revealed that Diana was one of several participants who had to be cut out of the study due to unexpected budget restraints. This disappoints Spencer, and he tries to find another study that his mother could participate in.
Though he is successful in finding another study, Spencer eventually decides to take her out of it and bring her to Quantico because the experimental medicine apparently didn't work. She now lives in his apartment. His attempt to find a caretaker fails, when Diana, in the midst of one of her episodes, floods the apartment and forces the caretaker out, accusing her of being a spy. She then states that she and Spencer "can only trust each other." Later, Spencer receives an old scrapbook Diana kept since high school. He then tries to persuade her into taking her medicine, but she refuses, stating that it tastes awful.
Spencer hires another caretaker, Cassie Campbell. Diana refuses to say hello to her, locking herself in Spencer's bedroom. Despite this, Cassie decides to take care of Diana. However, Spencer apparently lies to her about her medication. Diana is later seen flushing her medication down the toilet. Spencer gets upset with her, stating that the medicine cannot be replaced. Diana then screams at him that she wants to go back to Las Vegas, slapping Spencer. Afterwards, she goes to take a nap, eventually recovering from her schizophrenic episode. She sees her scrapbook, but has forgotten about its origins, as well as slapping Spencer. Looking through it, Diana recounts taking Spencer to the circus, but cannot remember who Elvis Presley is. Later, she completely forgets who Spencer is.
Diana visits Spencer at Millburn Correctional Facility. She tells him that Cassie got fired and replaced with a new nurse. Her schizophrenia kicks in when Diana tells Spencer that they have to escape and when she goes to the door, she thinks that she is locked in, but Reid tells her that he is locked up because of the murder of Nadie Ramos in Mexico. Then, Diana blames herself for Reid's incarceration. Later, Diana tells Spencer that he went to Mexico because of the medicine for her Alzheimer's. She apologizes to Spencer and tells him that soon Alzheimer's will make her forget who he is and not remember loving him. Spencer tells Diana that love is not a memory as it is so much more than that. At the end of the episode, her nurse enters the visitor's room and is revealed to be Lindsey. She tells Diana it is time to go. When Lindsey takes Diana out of the room, Spencer suddenly remembers that Lindsey was the one who framed him. However, by the time he remembers that, Officer Butler shuts the door in front of him. Diana was later revealed to have been abducted by Lindsey in the next episode.
While holding Diana captive, Lindsey gets a call from her accomplice Cat Adams to put Diana on the phone. While doing so, Lindsey burns her car and the corpse of a man she was forced to kill, and then steals the man's truck with Diana in tow. Later, Lindsey drives to the cabin of Lionel Wilkins, Cat's other accomplice; there, he helps take Diana's unconscious body inside before he is killed by Lindsey. Afterwards, the BAU show up, talk Lindsey into surrendering, and rescue Diana. Afterwards, Diana is reunited with Reid, and he promises to never let go of her again.
- Her favourite author is Margery Kempe. (The Fisher King, Part 2)
- It is mentioned that Diana used to smoke. (Memoriam) It is unknown if she still does.
- She holds lectures about literature to fellow patients at the sanitarium.
- She is terrified of flying. (The Fisher King, Part 1 and The Fisher King, Part 2) She appears to have since overcome this fear, but now has a fear of being on cruise ships. (The Storm) It is unknown if she always had a fear of cruise ships or if this fear developed at some point after she overcame her fear of flying.
- She went off of her medication while pregnant with Spencer; she was terrified every day, but said it was worth it. (The Instincts)
- She seems to share a sweater-jacket with Spencer, wearing it in The Fisher King, Part 1 and Memoriam.
- She keeps a Broadway playbill, from California Suite (June 1976-July 1977) at the Eugene O'Neill Theatre, in her scrapbook. (Surface Tension)