They both had secrets.

Maria Butina worked clandestinely as an agent of the Russian government with connections to Russian intelligence, U.S. prosecutors say.

Paul Erickson was a smooth-talking U.S. political activist who claimed connections to Republican bigwigs, but exaggerated his political ties and had a history of bad debts and fraud claims, interviews with people who know him and a review of court records show.

The couple, who were romantically involved, were together when Ms. Butina was arrested last week in Washington and charged as an unregistered Russian agent. Prosecutors say Ms. Butina, 29, sought Mr. Erickson, 56, as a conduit as she attempted to advance the Kremlin’s interests by leveraging their joint ties with the National Rifle Association and other conservative groups.

Prosecutors allege in court papers that their sexual relationship was a ruse on her part—“simply a necessary aspect of her activities”—and that she secretly “expressed disdain” for cohabiting with him.