Rosa was born in 1460. Nine months before, a Venetian nobleman paid a prostitute for her services, and Rosa was the result of the event. As the prostitute could not raise her child, Rosa survived in the streets of Venice alone.
At the age of fifteen, Rosa tried to steal from Antonio de Magianis, a stranger to her at the time, though she was caught. As the chief of the Venetian thieves guild, Antonio decided to raise Rosa as his pupil, rather than send her to the guards.
Throughout her apprenticeship, Antonio noted many of Rosa's talents, especially her high level of agility. Antonio also seemed to maintain a very parental relationship with Rosa, and would often call her sweetheart
In 1480, Rosa and several of her fellow thieves attempted an attack on the Palazzo della Seta, where the corrupt Emilio Barbarigo ruled over Venice's merchants. While her allies led away the guards, Rosa began to scale the building, but was hit in the leg by an arrow fired by an archer patrolling the palazzo.
Too injured to run, she demanded help from Ezio Auditore, who had been standing nearby, and who she had pickpocketed earlier that day. Despite this, Ezio consented, and protected her as they hurried towards the canals.
Along the way, Rosa proudly held her own, but she eventually collapsed and was unable to walk. Ezio thus carried her the rest of the way, until they met with Rosa's fellow thief Ugo, who placed her onto a nearby gondola. Ugo managed to ferry Rosa to the guild, while Ezio took out the archers pursuing them.
As she recovered from her wounds, Rosa taught Ezio how to grab higher ledges while scaling buildings. After Ezio's first successful climb-leap attempt, Rosa challenged him to race up the Santa Maria dei Frari within a set time, which he did successfully.
Years later, in 1485, Rosa aided Ezio and the other thieves in a second assault on the Palazzo della Seta. Ezio killed the archers guarding the palazzo, allowing Rosa and other members of the thieves guild to disguise themselves in their place. After Ezio successfully assassinated Emilio Barbarigo, Rosa and the other thieves shot down the rest of the guards, then she and Ezio opened the palazzo gates for Antonio.Rosa teasingly giving Ezio his birthday gift.
In 1488, on Ezio's birthday, Rosa gifted him with a shipping manifest from l'Arsenale, which detailed the return of a Templar boat the following day.