Antonia was an Italian pickpocket.
She and Mario stole Missy's dematerialisation circuit. Missy managed to track her down and dragged her into her TARDIS. Venice was then subject to various timeline changes caused by the portal that the Gryphons had created. Missy took her to 14th century Venice to find Mario as he'd fallen through but eventually discovered his corpse.
After Missy had corrected the timeline, she left Antonia in Venice in the past, but the Thirteenth Doctor went back to recover her, leaving her with a note, that chastised Missy for her lack of caution. (PROSE: The Liar, the Glitch and the War Zone)