In Biggles Buries a Hatchet, Fritz Lowenhardt was the nephew of Erich von Stalhein. A young man of seventeen, Fritz lived alone with his mother, von Stalhein's younger sister, in the Russian zone of Berlin. When von Stalhein knew he was about to be arrested soon, he sent Fritz to London to warn Biggles that the "Special Committee of the Secret Police" (probably of East Germany) had decided that Biggles should be liquidated. In a later book, Biggles Takes a Hand, von Stalhein would add that this task was given to the team of professional assassins headed by Ludwig Karkoff.
Heeding his uncle's instructions, Fritz sneaked away into West Berlin and with the help of a friend who was a pilot and who loaned him a pilot's uniform, managed to enter Britain without a passport. Making his way to Biggles' address which had been given to him by his uncle, Fritz delivered his message. He also told Biggles that von Stalhein had been arrested, tried for treason and sent to the island prison of Sakhalin. Before taking his leave, he suggested that Biggles might help his uncle escape.
On his way to his lodgings at the Brimsdale Hotel near Victoria, Fritz was stabbed in the back and narrowly escaped death. When Biggles told Gaskin about the background to the attack on Fritz, he was placed under police protection while at St George's Hospital and later at a convelescent home on the coast.
When the British authorities decided that von Stalhein should be rescued from Sakhalin, Air Commodore Raymond suggested that Fritz might be useful to the mission since he spoke Russian.
Fritz accompanied Biggles and his Air Police crew to Sakhalin where his knowledge of the Russian language indeed proved useful. He was able to befriend and communicate Ivan Miskoff, an ex-prisoner who provided a wealth of background information about prison routine to help Biggles plan the rescue. Fritz was also trusted by von Stalhein and his presence helped to break down any reluctance von Stalhein might have had in accepting help from Biggles.
At the end of the book, Biggles suggested that the Air Commodore might help in getting Fritz's mother out of East Berlin to Britain. Von Stalhein settled in London after his rescue but in Biggles in Mexico, we are told that Fritz settled in West Berlin with his mother. It appears, therefore, that the British authorities did help to get his mother to the West.