Umka was a Masena, a hideous tree-dwelling humanoid from the forests of Ladan. An unlucky fellow, he was captured by the Tarids one day because he was hunting prey near their territory, and they decided to sacrifice him to their Fire God (actually the Sun.) After weeks of solitude in the castle of Ul Vas, he was granted a cellmate, the Earthman John Carter, who had come to Ladan to rescue his beloved Dejah Thoris, who had been kidnapped (again.)
The man and the Masena spent their time getting to know each other while Carter tried to devise a plan for escape. Umka taught Carter the language of the Tarids, and how he could resist the hypnotic spell that rendered the Tarids invisible and inaudable. This way Carter was finally able to see the Tarids and effectively fight back against them. Carter later learned the Masena's eat people as well, and was surprised Umka had never attempted to eat him during their imprisonment.
Umka assisted Carter, Gar Nal, Jat Or and Ur Jan in their escape from the castle. Together they stole back the two space ships and freed Dejah Thoris and Zanda. When Carter was recaptured in an attempt to free Ozara, Umka escaped along with Dejah Thoris, Zanda, Jat Or and Ur Jan in Carter's ship.
The ship eventually landed close to Umka's homeland, and he was reunited with his people. When Carter and Ozara also ended up in the land of the Masena's after escaping the Tarids, Umka defended them against the Masena king and confinced his people to let the two go rather than eat them. Umka stayed behind in the forest when Carter and company departed for Domnia to bring Ozara home.