Onku was an earthbending orphan who lost his parents at a young age and resided at Grandma Lokai's Earth Orphanage. He was quite innocent and enjoyed company, referring to everyone as his friend. As such, he had a hard time letting go of people, as evidenced when he battled Aang in order to keep Toph, Sokka, and Katara at the orphanage.
After a vicious storm scattered Team Avatar throughout a vast forest, Onku watched as Aang navigated through the woods. He observed from a distance as Aang defended himself against a colony of viper bats and showed particular interest after Aang commented on how he must find the others. He petted a viper bat and set off to find Aang's friends. Not long after, he saw Katara and Sokka climbing up toward Grandma Lokai's Earth Orphanage and decided to assist them with earthbending. He then enthusiastically introduced the duo to Grandma Lokai, but they were more excited to see that Toph was okay. They proceeded to laugh until Grandma Lokai gestured for Onku to fetch tights and gowns for the newly arrived guests. Onku happily stated that the trio would remain with Grandma Lokai and him forever, when Aang entered, stating the contrary. He attacked Onku with an airbending blast, and Onku was quick to defend himself and his new friends. Katara, having realized that Onku was an orphan under Grandma Lokai's care, halted the fight. Grandma Lokai apologized for the confusion and proceeded to offer the gang beetle-worm soup.