Mike Stone is an agent with British Intelligence.
It is likely, since his actions take place during WWII, it is with Military Intelligence but since I know virtually nothing about this individual, that is speculation.
Interestingly, approximately five years before we are told of these two adventures of Mike Stone, one of which dealt with a "Secret Enemy" and the other with very large mechanized fighting machines apparently dubbed "Sand Spiders", there was released in another publication owned by the same company two tales nearly back-to-back about an agent codenamed who had to tussle first with a "Secret Enemy" and then with "Sand Spiders". Now only from the words about Spy 13 do we know that the first adventure dealt with finding a very powerful but very well hidden mole in British Intelligence, planted there years before WWII started, and that the second adventure had to do with a powerful weapon the Germans had for desert warfare. That said, I cannot say for sure whether Mike Stone had to go up against similar or the same adversaries and if they did, did one not get the job done well enough?
All conjecture, of course.
The only information we really have so far is that Mike Stone is a secret agent and he is either blond (as he appears in the first picture) or strawberry-blond (as he looks in the second). He certainly looks like he is in very good shape and gives off the aura that he can take care of himself in the second picture, though not so much in the first. Oh, he is right-handed, too.
I had to wonder, as I stared at these two adventure covers, how many other Spy 13 escapades Stone was taking credit for.