As a Fire Sage, Kaja was chosen to help Avatar Roku learn to control the Avatar State. Roku managed to trigger the state after five months by utilizing the power of the winter solstice sun rather than triggering it on his own. In doing this, he became trapped under its influence and could not control his actions, destroying most of the temple. Kaja eventually managed to free him from the Avatar State and ordered him to rebuild the temple, making it clear to the Avatar that his training would only be continued after the reconstruction. Not long after, Roku managed to master the Avatar State and Kaja's job was complete.
Following the Air Nomad Genocide, Kaja was the first to make a controversial declaration that the Avatar had not died in the attack. Some time later, Kaja fathered a son, who in turn fathered Shyu. Kaja's son fiercely defended his father's views and was eventually executed for defying the Fire Lord.