Kiyi was born to Noren and Ursa, who went through life as Noren and Noriko respectively after having their faces changed by the Mother of Faces.
Meeting Team Avatar
When Noren invited Team Avatar into his family's house in 102 AG, Kiyi decided to talk to Zuko and Azula, who were, unbeknownst to either of them, actually her half-siblings. She asked them if they wanted to see her doll, a suggestion Azula bluntly refused and Zuko happily accepted. Kiyi told him that the toy was also named Kiyi as she thought it was "such a good name". When Zuko commented on the doll's haircut, Kiyi told him that she had tried to make the doll prettier, but accidentally cut too much of its hair off. Zuko responded that Azula used to do things like that to her own dolls as a little girl, prompting Azula to clarify that she cut off their heads, rather than their hair. After she was asked whether she wanted to see a demonstration, Kiyi refused and went behind Zuko in fear while hugging her doll tightly to herself. Shortly thereafter, Kiyi explained to her father that she was trying to be hospitable to their guests after he entered with Sokka. When Team Avatar left her family's house, Kiyi ran up to Zuko and begged him to come back some time again.
Kiyi met Zuko again when he returned some time after, giving him a hug and insisting he have dinner with her and her family. After Zuko claimed her mother to be his own mother, Ursa, Kiyi grew confused and hugged her, though was passed to her father when they heard sounds coming from the roof. Before she could make sense of the situation, Azula barged into the family's home, causing Kiyi to scream for her mother. She was subsequently taken to safety by her father and Sokka as the princess fought with Zuko. Later, Kiyi and her father sat at a campfire, preparing a meal while her mother, who had regained her old face and memories, began talking about her past with Zuko.
Visiting the Fire Nation Capital
Some time later, Kiyi boarded the ship that would take her, her family, Zuko, and Team Avatar to the Fire Nation Capital for the first time since Ursa's exile. During the journey, she entertained herself by riding on the back of a flying dolphin fish along with Aang until Ursa realized the possible danger of riding a wild animal and called them back. As Aang brought her back on to the boat, she was pulled into a tight hug by her mother, who stated that she could not lose her. However, not recognizing Ursa as her mother due to the restoration of her old face, Kiyi screamed to be released. She ran to hug her father's leg and urged him to accompany her downstairs, which prompted Noren to pick her up and take her below the deck.
After having been made aware of a possible security threat, Zuko diverted course and arranged for his cruiser to rendezvous with a submarine, much to Kiyi's surprise. She met Iroh and Suki shortly thereafter, remarking to her half-brother that the Kyoshi Warrior was pretty. As she prepared to cross the plank to the submarine, Zuko suggested for her to accompany her mother as she might be scared. After initially refusing his request, Kiyi relented and agreed to take her mother's hand as a favor to him.
Kiyi and her family were transferred to a carriage after reaching the mainland and set course to the capital through a hidden route, where the caravan was ambushed by the New Ozai Society. Though Ursa urged Kiyi to remain brave, Kiyi insisted that she was not scared and recoiled from her mother's touch, complaining that her hands were too cold. After a brief altercation, Ukano and his organization were able to subdue Zuko and the Kyoshi Warriors, and prepared to burn the carriage in which Kiyi and her family were taking refuge. The fight resumed after Kei Lo freed Zuko and reinforcements arrived via hot air balloon, exciting Kiyi despite all the danger.
After the attackers were subdued, Kiyi and her family finally arrived at the Fire Nation Royal Palace, where she eagerly dragged her father along to explore the palace, despite Noren's concern that she should rest after an exciting afternoon. Kiyi brushed him off, saying that "Zuzu" would keep them safe, which surprised both Zuko and Ursa.
The next night while being walked to her bed with her parents, Kiyi saw ConstableSung escorting Ukano out of the palace and asked her father what was going on. Her mother assured her that it was nothing, but Kiyi yelled at her that she was "asking Daddy, not [her]!" and told her to let go as her hands were freezing. Ursa stayed back as Noren took Kiyi to her bedroom.
While Kiyi was asleep, Ursa rushed into her bedroom after inadvertently hearing from Suki and Ty Lee that more dark spirits had been spotted and. After seeing Kiyi still in bed, Ursa calmed down and hugged her, glad that she was safe.
Later on, Kiyi woke up due to her sleeping mother holding her too tight with "her cold, cold hands". To avoid waking up her parents, Kiyi whispered for Zuko, who carried her to the other room on his shoulders. On the way there, she asked him what he did that day and was informed that he had been dealing with a problem, though it was nothing for her to worry about as he would take care of it. After placing her in bed, Kiyi told "Zuzu" that she knew he would before falling asleep. While she was asleep, however, the Kemurikage impersonators came into her room and used sedatives on her to kidnap her.
While her brother and his friends attempted in vain to get her back, two of the Kemurikage impersonators got away with her and kidnapped another child before taking them both to the room with the other kidnapped children. As they got there, Kiyi woke up due to the sedatives wearing off early and began screaming at the Kemurikage impersonator holding her to let her go. Before the Kemurikage impersonators both left, she told them she felt sorry for them as her brother would find them and "beat them to a pulp".
When the children later heard a suspicious noise coming from behind a closed door, Kiyi told them all to be quiet, as she was trying to concentrate. Envisioning a firebending move she had seen Zuko perform in the woods when they were ambushed, she tried to mirror the action. Successfully tapping into her firebending abilities, she melted a hole in the closed doot, which Ukano subsequently doused with a bucket of water. While Ukano later escorted all the children out of their prison, Kiyi rejected Guri's marriage proposal. Before they could escape completely, however, their progress was thwarted by two of Azula's henchwomen, though Aang, Mai, and Kei Lo appeared and helped them out. Kiyi joyfully reunited with Zuko, and later that night, when she was asleep in the same bed as her father, she expressed her happiness over Ursa return, accepting her mother's new face.