Absolutely nothing is currently known about Cassie's background, other than she became a caretaker. She was hired by Spencer Reid due to the mental instability of his mother Diana, who was suffering from both paranoid schizophrenia and Alzheimer's disease. Earlier, Spencer made attempts to cure Diana of her Alzheimer's by entering her in groundbreaking clinical studies set up to combat the condition. Diana was one of the participants cut from the first study due to unexpected budget restraints, while Spencer chose to take her out of the second one because the experimental medicine apparently didn't work.
Cassie arrives at Spencer's apartment, where Diana is staying, and greets both of them. However, Diana refuses to say hello to her and just locks herself in Spencer's bedroom. Despite this, Cassie agrees to take care of Diana. As a precaution, she asks Spencer about any medication Diana is taking. He tells her about the medication he has been giving to treat Diana's schizophrenia and Alzheimer's, but refrains from revealing the experimental medication to her. She thanks him and leaves the apartment.
During a case in Palm Springs, California, involving the discovery of male torsos, Spencer calls Cassie, asking for an update on his mother's progress. Cassie tells him Diana is doing fine, then advises him that she would be in a better state if Spencer were home taking care of her as well. Distracted by his current task in finding the unsub, Spencer quickly ends the conversation and hangs up.
In the earlier episode, Cassie was shot and killed by Lindsey Vaughn, who then replaced her as Diana's nurse without Diana's knowledge, operating under the name of "Carol Atkinson". When the BAU get wind of Diana and Cassie's disappearances, JJ and Stephen go to Reid's apartment to investigate. They find the apartment empty and Stephen notes that Cassie's car is gone. JJ proposes that the two start looking for Diana by following her daily routine, which JJ memorized.
Sometime after, Alvez finds Cassie's abandoned car and contacts Garcia to get access to a nearby CCTV camera. Stephen and JJ also call Garcia to report that they have had no luck tracking down Diana. Just then, Garcia uncovers another of Lindsey's aliases, which she had used to rent an apartment in Reid's building for six months. Walker and JJ raid the apartment and find Cassie's body on the floor. Next to Cassie's body, they see a table with multiple photos of Reid, showing that Lindsey had been stalking him throughout the events of Season Twelve.