Marshall has a quirky and eccentric personality, which contrasts with the stern nature of those around him. He is also socially inept, having a tendency to speak very rapidly, often babbling about tangential subjects in the middle of technical briefings (before someone impatiently brings Marshall back on to the topic at hand). He is always dressed well and wears his hair in a slick sidepart. He has a fondness for unnecessary visual aids when giving a presentation, once even playing a full drum set to make a point (actor Weisman is in a rock band). However, his expertise in almost any subject imaginable is unparalleled, being well-versed in robotics, chemistry, acoustics, electronics, biometrics, explosives, computer networks and many other subjects. He also has a photographic memory.Like Sydney, he initially believes SD-6 is a black-ops division of the CIA. Because he is not a field agent, he often doctors photographs of himself in exotic locations to fool his mother into thinking he works for a company requiring a lot of travel; he once asked Sydney to take the photographs for said images. When SD-6 is destroyed, Marshall is brought into the CIA for real. Unlike his colleague Marcus Dixon, he adjusts to the change easily and soon becomes a valued member of the team. Marshall eventually meets an NSA agent with similar geekish tendencies, Carrie Bowman, and the two got married and had a child, Mitchell.Marshall provides some comic relief, but he has some serious moments when he is worried about those he cares about, including Sydney, but mostly revolving around Carrie and Mitchell. Marshall has also discovered how hard it is to lie to those he loves, having to keep his job a secret even from his wife.