In Biggles in the Gobi, Mr Ritzen was one of the refugee missionaries whom Biggles was sent to rescue from Western China. Ritzen, a Swedish missionary, had hidden in the caves of Nan-hu with 10 other colleagues.
When Nan-hu had been attacked by the troops belonging to Colonel Ma Chang, most of the missionaries had been captured or killed. However, Ritzen, Father Dubron, Miss Treves and Miss Summers had escaped capture and were on hand to meet Algy and Ginger on their arrival at Nan-hu.
Ritzen spoke English as well as the local languages and acted as the interpreter between Algy and the Chinese monks at Nan-hu as well as the Kirghiz horsemen who came to the oasis. He also had some medical training. After some Kirghiz horsemen had been injured in a battle with the soldiers of Colonel Ma Chang, Ritzen suggested to Algy that they should offer to tend the wounds of the Kirghiz as a way of winning their trust.
At the end of the book, Ritzen was among the people safely airlifted out when Biggles arrived at Nan-hu with the Halifax.