Tiger Lester is an agent with the British Secret Service.
It is British Military Intelligence that he works for when we first meet him, which makes sense since it is in the middle of World War II, and the branch of the espionage community he works for is M.I.3 - military intelligence gathering operations covering all of Western Europe, including Germany. As a result of being officially in the Army, Lester holds the rank of Captain.
Lester is a veteran operative having been active in the field already several years before the start of hostilities, transferred to the newly formed M.I.3 when his boss, Colonel Metcalfe, assumed the directorship of it. It was the same Metcalfe who considered Lester to be "one of the best Secret Service agents Britain ever had". And it was Metcalfe who would later admiringly chastise him with "You're a spy-hound. You sniff out a case a mile away. And then you're never happy until you're in the middle of it."
The actual first name of Captain Lester is not mentioned; everyone always calls him either by his last name, a common practice in the military, or by his nickname 'Tiger'. That came to him "not because at that time he had shown unusual courage, but because his school was famous for its rugby football, and the Leicester 'Tigers' were among their heroes". Still, many who have gone up against him, and some of his colleagues who have watched it happen, have decided the name "was not ill-applied".
Though his work is especially covert, classified at the highest levels, the results are known to those in control and very much appreciated. He is one of the few in his profession to have been awarded an D.S.O., pinned on his suit by the King personally in a private audience.
Lester is described, pretty blandly I would point out, as being "rather tall with a powerful build". There is not much more said about that except for his having "a fairly forgettable face which is good because 'the first essential [rule] of the Secret Service is that it should be secret'".
For over half of the adventures we have of Lester, he works largely alone, certainly without any long term partnership. That comes sort of an end in a matter dealing with Gibraltar where he needs the assistance of Midge Parker, a petite and very young-looking M.I.5 operative. From then on, the two find working together not only pleasant but also quite beneficial.