Although he does not appear in person in Biggles Foreign Legionnaire, Capitan Klein or Captain Klein was mentioned several times in the text. Klein, probably a German, was the commander of the secret squadron operated by the warmongering syndicate in the remote deserts of Northern Iraq. Like so many others in the squadron, he was also a deserter from the French Foreign Legion.

At first Klein had done his job efficiently. He maintained discipline with the help of his two lieutenants, Voss and Leffers and organised a steady supply of beer and cigarettes to keep up the morale of his squadron, which comprised mainly deserters and other criminal elements who were apt to be rebellious.

However Klein soon broke down probably under the strain and took to drink. He quarrelled with his deputies, Voss and Leffers and the men began to suspect that he was keeping the cigarettes and beer for himself. To prevent his men from complaining to headquarters, Klein took the valve of the base radio and kept it with him. Unfortunately, he still had the valve with him when he took off, still drunk, on a bombing trip which had been ordered, and crashed into a hillside.

Klein's actions unwittingly led to the demise of the secret squadron even before any intervention by Biggles and Ginger. His death deprived his unit of leadership as well as the means to communicate with the outside world. Discipline soon began to break down. By the time Biggles and Ginger arrived at the squadron, it had ceased to be an effective military unit.