Miss Lonely Hearts is the nickname for a client of the Dollhouse who regularly hires Victor, in one consistent imprinted identity, for engagements of a romantic/sexual nature. Dr. Saunders cautions against this type of repeated imprint, but the Dollhouse continues to acquiesce to this client's needs.
In the episode "A Spy in the House of Love" it is revealed that the identity is actually an alias used by Adelle DeWitt, who arranges for Victor's handler to deliver him to the home of an elderly woman as a deception. After arrival, Victor, as suave Englishman Roger, drives a car to a rendezvous with DeWitt, who answers to "Katherine" when with him.
Coincidentally, "Lonely Hearts" is used elsewhere in Whedon's work. It is also the name of episode 1x02 of Angel.