Mark Castile is an agent with the RCCIS.
That is the Royal Canadian Counter Intelligence Service [it is sometimes shown without the trailing 's']. He has been with them for some number of years and has had an impressive number of successes such that the head of the organization, the Rt. Honorable Joseph C. Samuels of Her Majesty's Service, is familiar enough with his record that for the first recorded mission we have of Castile, Samuels asks for him by name. This is interesting to Castile as we are told by the operative that up until the meeting giving him his next assignment, he had never met the man in person; to Castile up til then he was just Mr. Big.
It really should be Castile who would be known by that sobriquet for Castile stands an impressive 6'6" tall and weighs 260 lbs with very little of that not being muscle. In that meeting with his boss, Castile was forced to sit in a straight-back chair where only the "center portion of his buttocks" fit on the pad. "The way he sat hunched on his chair he looked like a football player" something he had been at one time.
"Upon graduation from law school, he had entered professional football as a means of financing a law office in his home town of Whitehall, Ontario". His playing days were ended with a hitch in Korea, Tokyo and Saigon after which the Ministry of National Defense 'traded' him toe the RCCIS. The military had found that he was "too big a target for active warfare" so he got the transfer to the Intelligence department "where his knowledge and skills were put to good use".
Samuels has chosen Castile for this new mission because he was considered uniquely qualified. Castile is not so convinced because the job would involve investigating a series of murders in South America of Western operatives and Castile points out succinctly, "I'm a hit man. My job is to wipe out who, where, and when". When he is told his job was to be both, Castile realized, and stated as much, that he was being used as bait. A telling point about Castile is that he had absolutely no problem with that.
Now it is time to point out that Castile is not this operative's real name; it is a cover that he is given for this mission and one he will keep. What his original name is we do not learn and it is not important since he stays being Castile. Whether it was that the time between the two missions we are able to follow him on were so close together he had not time to change identify or he just liked being Castile, we never learn.
Physically, in addition to his large stature, we are told about his having a "huge bulk with grey eyes that looked like steel from the frame of black wavy hair above the straight mouth, the straight nose, the clefted chin", all resulting in a "handsome countenance". Certainly his size makes sure the ladies he encounters, some of whom want to both love and kill him, notice him right away. He is rather difficult to miss.
Something else that is sort of hard to miss when reading these two adventures is the carnal, lustful, and explicitly detained the physical relations (sex, obviously) between Castile and the women he encounters, not to mention those women and other people. He and they like sex. A lot!