Marcia, along with her mother and little brother Max, lived in a town that was being threatened with massacre by Raiders. When the Brotherhood of Steel came into town, Leila Rahmani decided to stay and fight the Raiders, while Daniel Shin decided to arm ordinary civilians for protection against the Raiders, which violated the Brotherhood's protocol to safeguard them instead of distribute them. The Raiders were able to steal their weapons and slaughtered everyone in the town, including Alan Connors, except for Marcia and Max. The orphaned siblings became refugees and traveled with the Brotherhood to Fort Atlas. Marcia's mother owned a necklace and Marcia regrets not being able to retrieve it from her body as a memento.
Unlike her brother, Marcia blames the Brotherhood for their parent's deaths because the Brotherhood supplied the weapons. She feels the Brotherhood should have never fought back against the Raiders to begin with, blaming both Shin's and Rahmani's actions. She feels the Brotherhood are reckless, and justifies it with some "bullshit savior complex" and is incredibly irritated by how her little brother idolizes the Brotherhood. In her diary, she called the Brotherhood "assholes" and expressed her blame. She even stated that she is considering joining the Raiders at the Crater, a different group of raiders not involved with her past tragedy, feeling that at least they look out for their own people and try to live their lives independently, free from the "oppression" of the Brotherhood. Although Marcia wanted to leave Atlas because watching the Brotherhood follow orders was a constant reminder of her mother's death, she stayed as long as she could because Max refused to go. She got along with Odessa Valdez and Luis Ramirez, however.
In 2104, after Ramirez tells Marcia that he would never let anything happen to Max, Marcia decides to leaves Fort Atlas. She tells Max she will always love him, which weirds him out because it is extremely unusual for Marcia to act like that. Although Marcia was sent to Foundation to get some supplies, several Brotherhood initiates returned saying they saw no sign of her there. She joins the Raider's War Party, becoming fast friends with Burke, Pierce and Sheena, considering them to be her new family. Meeting the Vault Dweller, Marcia decides to go look for Sheena and Burke at the AMS corporate headquarters after they disappear. They find clues such as Burke's dove necklace and they fight Kit, a member of the Hellcat Company at the top of the building. They eventually find Sheena and Burke held captive in the basement.
Find the kids in Fort Atlas: This is an unmarked miscellaneous quest that may appear when a Brotherhood member wonders how the children they saved are doing. To find Marcia and Max, pass through the laser grid to get to the main area and head into the room on the right without going up the stairs. There is a door here which leads to the basement area.
For Marcia to return to Fort Atlas, the Vault Dweller must be kind and supportive to Marcia and empathize with her, instead of telling her things like taking her mother's necklace would mean nothing, and that death is a reality in the wasteland. Being rude to Sheena and Burke will also encourage Marcia to not return. Asking Marcia what she would think if her mother saw her if she left Max alone will encourage Marcia to return to Fort Atlas. Marcia realizes that she values her connections with some Brotherhood members, such as the Vault Dweller.
If returned to Fort Atlas, Marcia will appear alongside Max in Fort Atlas at the end of The Catalyst. She comments that she is actually starting to enjoy her time in Fort Atlas. She thanks the Vault Dweller for helping her see the perspective that the Brotherhood is not solely to blame for the death of her parents. If Rahmani leads the Brotherhood, Marcia considers her the best chance the Brotherhood has to make a truce with the Raiders, though says her self-righteous attitude and inability to "live and let live" could interfere with that. If Shin leads the Brotherhood, Marcia will be dismayed a "militant asshat" is in charge.
If the Vault Dweller fails to convince Marcia to return or encourages her to stay with the Raiders, she will stay with the Raiders in the War Party room. She will wonder how Max is doing and says she misses her friends at Fort Atlas, but says being a Raider is awesome. Marcia is also hoping Pierce can understand her point of view, stating that she would never be able to hurt any of the Brotherhood members. Depending on how the Vault Dweller has interacted with her, she is either friendly or treats them with disdain.
↑Vault Dweller: "Why are you so rude?" Marcia Leone: "I'm sure you wouldn't be too thrilled to talk to anyone if both your parents were dead and you're stuck with the people who caused it." (Marcia Leone's dialogue)
↑Vault Dweller: "How did they cause their death?" Marcia Leone: "I really don't want to talk about it. Just watch out, the Brotherhood can be reckless and they justify it with some bullshit savior complex." (Marcia Leone's dialogue)