Mark Brodie is a good friend of Orlando Smith who wants to apply for a job in Varrock Museum. Only after his friend's death at the hands of an automaton guardian during the events of the Assassination of Guthix does Mark receive employment at the museum. Ironically, he becomes Orlando's replacement. After that event, he is located at the basement floor of Varrock Museum in his place.
His name and clothing are almost certainly homage to the fictional character Marcus Brody from the Indiana Jones film series.