Marsilia is a 14-year-old daughter of minor nobles from Rome who enjoys reading and writing and is far too inquisitive. She likes asking her mother's guests about their journeys and the gossip from their courts, traders about the places they've come from, and family members about their businesses and thoughts about the state of the papacy. She's a exceptionally pretty, charming young girl who tends to get away with being too talkative and intelligent, particularly since she'll always offer to run an errand with members of her father's household and can be trusted to oversee small tasks for those who will give them to her. She's also quick to come up with the latest gossip to share with the family, since she's often asking questions and charming answers out of people who find her quirky and harmless (or eavesdropping on conversations she shouldn't have a part in). Her father allows her to do some things in his stead as a son would, perhaps because he has none, perhaps because she's handy, or perhaps because she often acts boyish; some people in the family treat her the same way, while others are baffled by Mazzeo's reticence to send her back inside.
Marsilia maintains a written correspondence on behalf of her mother with a great-uncle acting as a cardinal in Avignon (among others), and is deeply disturbed by the corruption implied in his letters and the complaints of her uncles and other family members about the effect the moved papacy has had on Rome and the Colonnas. She is faithful, but has no intentions of devoting herself to a holy order; she sees how they are made of men and can so easily be led astray. She is more pliable to the authority of her family and will take on what tasks she can to help them, for those of them aware enough to understand that she's no use at "women's work." Her good looks, charm, youth, and a lack of worry about her marrying (since she doesn't lack for money, nor is she the last to remain unmarried yet) allow her to get away with a lot.
She also maintains a rivalry with an Orsini boy of her age, Ruggieri, who has a crush on her that he suppresses by pitching olives at her head. Unfortunately for him, she enjoys olives and has much better aim.