In Biggles in Borneo, Captain Rex Larrymore was an ex-R.F.C. officer who approached Air Commodore Raymond with a proposal to set up a secret strike base in the remote mountains of Borneo.
Biggles' first meeting with Rex served as another occasion for one of Johns' exquisite pen pictures. Rex was described as a "small, keen-faced, sun-burned man" whose most outstanding features were a head of close-clipped red hair and bright, almost brilliant blue eyes. Biggles noted that perhaps the colour of the eyes might have been emphasized by the hair but that he had never seen eyes so piercingly blue. Rex had served as a pilot during the First World War. When peace came, he decided to put his flying skills to good use, flying to remote areas to take up prospecting for diamonds and gold. During these expeditions, he chanced upon a dry lake bed near Mount Mulu in the highlands of Borneo. Discovering there were deposits of diamoniferous gravel there, he set up a private landing strip and prospecting base there called Lucky Strike and over several years, built up a stockpile of over two thousand gallons of petrol and oil. During this time, he had also befriend the native Punan tribe who lived in a jungle village nearby, headed by their chief Suba.
When the Second World War broke out and the Japanese forces invaded Borneo, Rex fled the island and made his way via Australia to Britain where he offered "Lucky Strike" to the Air Ministry as a potential strike base.
When the R.A.F. took up his offer, Rex accompanied Biggles and his squadron to "Lucky Strike" chiefly to act as interpreter between Biggles and the native Punans who were to become invaluable allies. Rex accompanied Suba and his warriors out on several jungle expeditions, first to rescue Jackson, Bill Gray and Pat Flannagan. Later in the book, he also went with Suba and his warriors to attack the troop column of General Yashnowada which was moving up to attack "Lucky Strike".
Rex also accompanied Biggles on various missions, wherever Suba was taken a long for his expert local knowledge. These included a reconnaissance mission to Talut Island to find out what happened to Algy and Ginger, and the raid on Cotabato to rescue Allied prisoners of war.
At the end of the book, when Biggles' 666 Squadron was being relieved by several Australian squadrons at Lucky Strike, Rex chose to stay behind. He told Biggles that Suba had made him his blood brother and in any case the newly arrived squadrons would need his help.