Marissa was born in Georgia, where she grew up as the younger of two children. She and her older sister, Paige, enjoyed a relatively privileged upbringing as the daughters of a prominent doctor in the community, and Marissa figured she would do what her mother had done with her life: specifically, go to college, get a professional degree, then marry either a doctor or lawyer and become a well-to-do housewife in the country club set.
All this was turned on its head when she found out she was a witch and went to Sonora. Somewhat introverted at the best of times, she struggled with the unfamiliarity of her surroundings and the culture therein, and with discovering that some of the student population looked down on her simply because her parents were not magical. This problem was exacerbated when she proved to have difficulty with summoning the magical power to do many spells, though she somewhat offset this problem by assuming the social role of the Seeker for the Crotalus Quidditch team.
Parents: Dr. James Stephenson and Elizabeth Stephenson (deceased). Non-magical.
Siblings: Paige Stephenson. Non-magical.
Husband: Andrew Duell. Magical.
Daughter: Valentine Duell. Magical.
Parents-in-law: Marshall Duell (deceased) and Gwen Duell (deceased).
Sister-in-law: Giselle Duell. Magical.
Cousin-by-marriage: Jhonice Trevear. Magical.
After Sonora, Marissa struggled for several years to get her feet under her. A chasm had grown between her and her parents and sister due to her frequent long absences, she felt somewhat aimless, and less than a year after she left Sonora, her boyfriend's life nearly fell apart after a freak accident led to the deaths of both of his parents and the blinding of his infant sister. The university years are, on the whole, not pleasant memories for her, though life began to look up afterward: she got married, settled down, eventually had a daughter, and moved to Greece for a fresh start.
Or at least, that's how she remembered it for about seven years.