Matt Stewart is an agent with British Intelligence.
His full name is, obviously, Matthew, but people either call him Matt or, if friendly, 'Crash', a nickname given him after a particularly rough game of rugby and several cracked ribs. They occurred in "a couple of thumping tackles in the same game. Not my ribs you understand but those belonging to the other two blokes, who just about survived being mullered, following a fair bit of treatment and a few months off work and sport."
According to Stewart, he is 6'4" tall and weighs around 18 stone (252 lbs.). He also states that he is all muscle with no fat to be seen anywhere. He says this, though, right after his boss tells him that he was "putting on too much weight and getting a little slow", no longer being fit for the job and off for a 3-month intensive exercise plan. That part is almost certainly a ploy to give him cover for this next mission because, as he tells us at great length (pun intended) he would later that evening (and night and early morning) enjoy the company of a beautiful woman named Sophia Winter.
Ms. Winter will turn out to be far more than just "an energetic blonde" who brings along her own packet of twelve items of protection and who points out to Stewart how many have been used as the evening progress as well as how many remain available. She will soon let Stewart know that she is his new boss. Apparently she did not share the opinion of the woman she replaced that Stewart was in unsatisfactory condition.
Stewart has plenty of excitement in the field as well. He is definitely a very friendly guy who likes to have a good time but he also likes to accomplish him missions and he prefers to be able to walk away from them afterwards.