|Portrayed By||Renee O'Connor|
|Series||Xena: Warrior Princess|
|First Appearance||Deja Vu All Over Again|
|Last Appearance||Soul Possession|
|Residence||The United States|
|Affiliations||Harry O'Casey, Annie Day, Xena, Gabrielle, Joxer and Ares|
Mattie Merrill is a psychologist and past-life therapist living in Southern California in the late 20th century. With Marco, she ran Who You Used To Be, a clinic where people paid to be put under hypnosis to remember previous incarnations of themselves. Mattie developed the techniques used for the clinic's past-life regressions, but she believed them to be elaborate hoaxes designed make money while maybe giving some of her patients a bit more self-confidence.
When Harry O'Casey and Annie Day arrived at the clinic hoping to delve into Annie's belief that she used to be Xena, Mattie indulged them. Over the course of Annie's therapy, however, she learned that she was actually Joxer. This set off a chain of events prompting Mattie to discover, to her shock, that not only were her methods successful in recovering past-life memories, but that she was the reincarnation of Gabrielle. It was also revealed that Harry was the reincarnation of Xena and Marco was in fact Ares (literally). After Harry bested Ares again, Mattie entered into a relationship with Harry. (Xena: Warrior Princess: "Deja Vu All Over Again")
A couple of years later, after returning from their honeymoon in Greece, Mattie and Harry were tipped off by Annie that an ancient contract binding Xena to Ares had been uncovered and was on display at C.H.A.K.R.A.M. The pair investigated and ran afoul of Ares again, in the process learning more about a missing time in Xena's timeline. Although Ares was tricked into destroying the contract, he meddled with Xena and Gabrielle again by swapping Xena and Joxer's souls into the "correct" bodies, further complicating Mattie's personal life.(Xena: Warrior Princess: "Soul Possession")
- It is not clear if Mattie was Gabrielle's descendant as well as her reincarnation, or if she was also related to Janice Covington.
- Mattie retained her long hair even after regaining Gabrielle's memories of having shorter hair.
- There is considerable debate in the fandom about the implications of Mattie's eagerness for a relationship with Harry.