Sulpicia is not featured heavily in the series. She only appears at the end of Breaking Dawn, but does not say anything. In the book, during the confrontation between the Volturi and the Cullens, there appeared to be "two hooded figures, definitely women, flanked by two hulking bodyguards". These were Sulpicia and Athenodora. They are important to the ancients, and therefore are heavily protected. It`s not often that the wives are leaving the tower, not even during the southern rebellion, when the Romanians tried to overthrow them or when the Volturi were hunting the immortal children, like Jasper said in Breaking Dawn.
Added by I♥CarlisleCullen
Aro is Sulpicia's mate. Not much is known about their relationship except he keeps her heavily protected in the rare times she leaves the tower. The Romanians recognizing the wives in Breaking Dawn suggest they were together before the Volturi came to power. It is unknown when or how they met.
Athenodora is Caius's wife, with whom Sulpicia spends the majority of her time.