|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
Mary is a little girl from the Secret Vault. She helps the Initiate after his first fight with the general of the Super Mutants who attacked the Vault, Attis, where the Initiate gets their arm cut off by Attis who then pushes them into a hole leading to the Ruins. Here, Mary found the Initiate seriously injured and guides you to The Garden.
She often searches the Deathclaw infested ruins section, looking for her mother's lost ring. The Initiate could help her find it. Later, she escapes the Secret Vault when the auxiliary vent is off, allowing the people to enter the wastes.
Mary appears only in Fallout: Brotherhood of Steel.