Marshall (Frederick Weller), Mary's partner, is a fifth generation US Marshal. There was never any question about what he'd do with his life. Despite this laser focus on his career track, Marshall is by no means limited in his intellectual life. He knows a little bit about everything, which comes in handy when he and Mary are working a case or when they could both use a little comic relief as a distraction.
Despite being polar opposites, the two make a great team. As much as Marshall can sometimes get on Mary's very last nerve, they have a bond that transcends a traditional coworker relationship, having each put their lives on the line for each other.