Governor of New Ozai
Ukano successfully lead the Fire Nation through their conquest of Omashu, and as a reward for his accomplishments, was appointed Governor of the city, renamed New Ozai by Azula in honor of her father. He moved to the city with his wife Michi and their children Mai and Tom-Tom.
Some time later, Team Avatar helped the Omashu Resistance escape Ukano's rule, feigning illness. Fearing a plague, Ukano opened the city gates to let the "infected" citizens out of the city, while Tom-Tom followed them from behind. Thinking Tom-Tom was kidnapped by the Resistance, Ukano arranges a trade; his son in return for King Bumi, the original ruler of Omashu and Aang's childhood friend.
The trade was almost successful until Azula, after berating Ukano for failing as governor, took over the deal, along with Mai and Ty Lee, her two childhood friends. Despite the trio double-crossing Team Avatar and King Bumi willingly staying behind to wait for the right moment to strike back, Aang secretly returned Tom-Tom home anyway, disappearing before Ukano and his wife rejoiced at their son's return.
Founding the New Ozai Society
Months later, on the Day of the Black Sun, with the moon blocking the Sun and thus rendering firebenders unable to bend, Kimg Bumi successfully reclaimed Omashu. Ukano lost control of the city and returned to the Fire Nation Capital.
Following the end of the Hundred Year War and Zuko ascending to the throne, Ukano was offered a desk job in his new government, but declined on the belief that it was merely a way for Zuko to keep him under control. Viewing Zuko as a tyrant and Ozai as a capable Fire Lord, Ukano founded the New Ozai Society, bringing his son to the meetings to indoctrinate him into the life of the old Fire Nation. Knowing his daughter had recently broken up with Zuko and had information on his weaknesses, Ukano sent Kei Lo, another member of the society, to bring Mai to the meeting under the ruse of a date. Mai was not only shocked to see her father founded such a radical organization, but also angered that he would allow Tom-Tom into that life. She overpowered the other members who tried to stop her from leaving and left with Tom-Tom, but not before ignoring her father's pleas and punching Kei Lo. Following the incident, his wife left him because he cared more about politics than the safety of his children.
One night during the following year, Ukano received a late night visit from the "Kemurikage", who berated him for his past failures. Ukano expressed disbelief at their presence, believing them to be mere fables, but took their warning to heart as he held a meeting for the New Ozai Society the following day, preparing to dethrone Zuko. The New Ozai Society attacked Zuko and his family, but Ukano was stopped by Mai. After warning of Zuko's treachery, Ukano flees the scene. The "Kemurikage" approach him again, warning him that he will lose everything he holds dear if he fails, which later come true when they abduct his son Tom-Tom.
Ukano later barges in on a meeting between Zuko and Aang, berating Zuko for his failure to keep the Fire Nation safe while also chastizing Mai for taking Tom-Tom away from him, arguing that if he had stayed with him, the Kemurikage would not have abducted him. As Ukano is escorted out by the kidnappings' lead invetigator, Constable Sung, Ukano warns him to keep an eye on his own son, as Zuko's cowardice would get him kidnapped.
When Sung's son ends up being kidnapped by the "Kemurikage", he turns to Ukano for help, persuaded that he cannot wait for Zuko to take action. Ukano leads the New Ozai Society, now renamed the Safe Nation Society, to enforce curfew for human citizens against Zuko's orders. The "Kemurikage" show up attempting to kidnap another child, before they were stopped the the Safe Nation Society. Sung is suspended from his post and Ukano is order to disband the society. He briefly cooperates, but not without warning Zuko's leadership will bring ruin to the Fire Nation.
Ukano is then revealed to have been cooperating with the "Kemurikage" by hiding the kidnapped children away from the public, including his own son. He assures Tom-Tom that they are all playing a great part in restoring the Fire Nation to its glory, although it is clear even to Tom-Tom that Ukano is still afraid of what the "Kemurikage" will do to him.
Turning against the Kemurikage
Ukano is later told by Azula, who was leader of the "Kemurikage", to hold out and keep the kidnapped children inside their quarters until further notice. However, Ukano soon realizes that not only are the children expendable to her, she also has no intention of putting Ozai back on the throne. Ukano then releases the children and starts to lead them out, but is stopped by two Kemurikage imposters. Aang defeats them and allows Ukano and the children to escape. Ukano is later arrested by authorities for his role in the uprising against Zuko.