Burt Kennedy is an agent with the British Secret Service.
The time period of the activities of this operative is during the Second World War although exactly when is not known. Also not totally determined is whether the organization that Kennedy belongs to is a civilian-run one or actually part of military intelligence. It is likely the former but his ability to call up on the latter for assistance is noteworthy.
Kennedy is described more than once as a "young man" though how young he is is not known. He cannot be really young because it is shown by the narrator as well as the reaction of a deep-cover German spy who encounters him that his name is already well known to the opposition, enough to have "struck terror to [the man's] very soul" when Kennedy identifies himself (that and the fact that at the time Kennedy is seated at the man's dining table with two handguns pointing at him).
Kennedy is referred to as "Great Britain's master-spy, the one man above all others the Nazi regime wished to see dead". He likely earned this distinction through a series of successes aided by his mastery of disguise. He is shown to have the skill to assume the identities of other people, such as a captured German agent, going so far as to use makeup and prosthetics to look like the real person. With this impressive ability we have to assume that he is a native German speaker as well.
In addition to his makeup skills and his demonstrated strong intellect, Kennedy enjoys very quick reflexes and lighting reactions times coupled with superb agility making him a force to be reckoned with in a fight. And based on his reaction to the likelihood of such encounters, he relishes them.