To go along with themythos theme, Mary is more than likely based on , the creator of the mythos itself.
- Devil Summoner: Soul Hackers: Supporting Character
Mary appears before Protagonist and Nemissa when they enter Hotel Goumaden and forbids them from entering unless they had a reservation. When her master learns they have a COMP, he orders her to let them in whenever they want, which she complies.
When the protagonist speaks with Mary, he begins a long side-quest for her understanding of what it is to be human, more specifically, finding out what "love" and having a "heart" is. Near the end of her quest, Dr. Victor sends the player out to go do an errands with Mary, first going to the Automater, a shop that sells dolls. The owner at the Automatar says that she and all her friends (referring to her dolls, named Elizabeth and Juliet) are happy to meet Mary who greets them. Nemissa says that Mary doesn't have to do that, because dolls aren't actually alive, having no heart or soul. The owner says that Nemissa's rude and makes a remark about them being alive if someone cares about them.
Next, the trio head to EL-115. After Mary does her business, she asks Alpha and Beta, the owners of the store, if they can make her a "Heart." She says that Victor once told her that if she found someone who was important to her (and someone whom she was important to), she would gain a "Heart." Even though she has the protagonist and Hitomi, she still has no "Heart." Mary does not understand the concept of casual acquaintances. Nemissa apologizes for calling Mary a doll earlier, and for other such disparaging remarks (dolls have no souls, etc.), which Mary says that it's fine, since she appreciates Nemissa's bluntness. Alpha later explains that one cannot just give someone a "heart", a heart is judged by how much one is loved by others, not by how much one can love. With these experiences being done with, the trio go back to the Goumaden. Later if the player talks to Mary, she thanks the protagonist, smiles and says that she's somewhat understands what it means to have a heart.
She gives the protagonist access to Muramasa in the Goumaden, so the player can perform sword fusion.