He was shown in a flashback with his son and his wife on a boating trip where Zack got scared by a fish that landed on their boat ("Murphy's Lard").
He, Zack, Milo and Milo's dad went on a fishing trip on Murphy's Yaw. At first, he doesn't believe in Murphy's Law, but throughout the day he eventually believes in the Murphy family curse ("Time Out").
He has dark skin and brown hair and a goatee. He wears a blue cap, a short-sleeved blue shirt, and beige shorts with brown sandals.
He seems to be easy going and calm under pressure, as shown during his and Zack's fishing trip with Martin and Milo. Initially, he was skeptical of the existence of Murphy's Law, attributing the mishaps to things like 'inexperienced casting'.
The image gallery for Marcus Underwood may be viewed here.