Mariotto is the youngest of the stable boys working for the Duchessa of Bellezza. He is introduced in City of Secrets as the primary caretaker of the Duchessa's African cats. As a stable boy, he lives in the stables with the cats.
Unlike most of the Duchessa's servants, Mariotto does not defer to her and often sulks at how she spoils her African cats, especially because they will grow fat without sufficient exercise from all the food she tends to give them. Rather than being offended, Arianna finds his behavior refreshing and she trusts him to take good care of her cats. The young boy loves the cats as though they were his own and enjoys taking them out on runs through the swamps outside of Bellezza's city proper, often for several hours. The African cats respond well to him and are always well-behaved when he takes them for daily walks in the city.
While caring for the cats, Mariotto notices when the Duchessa begins bringing her maid Barbara to spend time with the cats as well. Though he eventually overhears how Barbara's time with the cats are to enhance her ability to become a convincing double for the Duchessa, Mariotto remains silent on the issue.
Because he spends a great deal of time with the Duchessa's cats, he notices right away that the first pair given to her by Niccolo di Chimici are a male and female. Despite his repeated assertions to Arianna that name "Florio" is inappropriate for the female, she does not believe him until the female is pregnant. After the renamed "Flora" gives birth to four kittens, Mariotto is charged with looking after the kittens as well, though most of the kittens are given away to the Duchessa's friends and acquaintances.