Lovelace seems to have an army of friends willing to dress up in disguises to help him seduce Clarissa, but Captain Tomlinson is the most persistent. Or maybe he just likes playing spy. Tomlinson lurks in the bushes by Mrs. Sinclair's for hours just to convince Clarissa that he's working for her uncle. It seems to pay off, though, because Lovelace invites Tomlinson back to play his role again and again.
Lovelace sure seems to trust this guy: "I doubt not your dexterity and management, dear M'Donald, nor your zeal" (289.1). If anything, the dude seems to be living vicariously through Lovelace and his crazy seduction schemes. Oh, and Lovelace throws him a few guineas here and there to keep him fully invested.
But the guy doesn't seem totally heartless. After Clarissa implores to his conscience and honor, Tomlinson seems "uneasy, he shifted his feet: her eye was upon him; he was therefore, after the rebuff he had met with, afraid to look at [Lovelace]" (244.19). Tomlinson may have second thoughts about being such a jerk, but he doesn't seem concerned enough to stop the plotting.