Rufilla is the wife of Salvius. Fans of the Cambridge Latin Course enjoy telling... stories about her, perhaps a little too much.
Okay, WAY too much.
The books don't tell much about Rufilla's background (some people just have to infer), except for the fact that she is related to Quintus. Rufilla has a tendency to complain, lamenting in Stage 2 over the isolation of an expensive country house of her choice, funded by Salvius after he grew sick of hearing her complain about her crowded life in the city. Because of this feeling of loneliness, Rufilla later invites Quintus to stay at Salvius' house.
As such, neither Rufilla nor Salvius appears to be happy with their relationship.