From Guild of Archivists
Race Stoneship

Branch was a humanoid who lived in the Stoneship Age. His name was given to him by Emmit, and the two quickly became friends. They lived together on "The Rocks", sharing the largest rock as a living quarters, and swimming and fishing in the sea. When Atrus first visited Stoneship, Branch and Emmit were still young adults. Along with others who arrived after him, Branch lived on The Rocks for at least another 10 years, as Atrus recounts meeting him again on a return visit.[1]

By the time the Stranger arrived on Myst and began to uncover the history of what had happened there, Branch and the other people who lived on The Rocks were gone. It is unclear whether they fled when Sirrus and Achenar began to exploit their father's Ages, or were subjected to the brothers' destructive behavior.