Mark's physical appearance reminds Bryon of "friendly lion". He is blonde and has "strange golden eyes", and has deceptive strength - "he is stronger than he looks" (Chapter 1). There is also something menacing about him, an energy barely restrained, that is reminiscent by the end of the book of "an impatient, dangerous, caged lion" (Chapter 11).
Mark's "golden" quality might also be used to describe his personality. Until Bryon turns him in, Mark is virtually always positive, energetic, and upbeat, almost blithe, like sunshine. Also, he has the maddening capability to charm even the most hardened soul, and the ability to get away with just about anything with a facility that might be described as "golden".