Leffers was one of the senior pilots of the warmongering syndicate in Biggles Foreign Legionnaire. Described as a young man of dark complexion, a hard face with a small scar on his cheek bone, Leffers, a German, was a pilot in the Luftwaffe during the Second World War. He went to the United States after the war where he killed a man and then found refuge in the French Foreign Legion. He deserted the Legion and joined the warmongering syndicate.
By the time of the events narrated in the book, Leffers had already risen to a senior position as one of the deputies of Captain Klein, the commander of the warmongering syndicate's secret squadrron. He must have been one of the first few to join the syndicate--the number on his Aladdin's Lamp device was 29, considerably more senior than Biggles and Ginger who were assigned number 123 and 122 respectively.
Biggles and Ginger first encountered Leffers at the Aladdin's Lamp night-club in Alexandria where he told them that he would be flying them out to the location of the secret squadron the following morning in his Beechcraft Bonanza. However Leffers declined to tell them where they were going.
After parting with Leffers, Biggles and Ginger returned to their hotel, the Continentale, and found the place in darkness. Searching the place, they stumble upon Cy Lindsay, an ex-syndicate member with a personal grudge and out to kill as many senior figures of the syndicate as possible. Lindsay had just killed Johann Klutz and was telling his story to Biggles and Ginger when Leffers discovered them whereupon Lindsay shot him dead as well.
Biggles was inclined to see Leffers' death as a setback, probably because this eliminated his chance of finding the location of the secret squadron. However, in going through Leffers' pockets, he found a map tracing with compass courses. This, together wtih Lindsay's description of the route allowed Biggles to go there himself.
Although Biggles did not know it at that time, the murder of Leffers would be the final death knell for the secret squadron. The commander, Captain Klein had died recently and Voss, the other deputy, had also died. With Leffers also gone, there was no other effective leadership for the squadron.