Origin
As a child, Kajak Thoris was kidnapped by Zodangan noblemen who were disgraced by Mors Kajak. His parents believed him to be dead. The noblemen raised him as their own. By letting him know his true name and birthright, they wanted him to feel every bit of indignity possible. Years later, Kajak was able to escape, killing his kidnappers in their sleep.
Coming back to Helium, he saw his younger sister Dejah Thoris being worshipped by his people. In fact, she had the life he should otherwise have. As a means of revenge, he kidnapped her and brainwashed her into believing she was a barmaid from Zodanga. One day, she secretly stopped her intake of medicine. Clear thoughts undid the brainwashing and she exposed her brother. He told her he had poisoned their mother, before escaping.