On Ji was a young student at a school in the Fire Nation. Her parents valued her education and hoped that she would succeed in school. They wished for her to further her studies at a good university in the Fire Nation Capital.
Aang, under the alias of Kuzon, came to the Fire Nation school, claiming to be a new student from the Fire Nation colonies. After school ended for the day, Aang met On Ji, who noticed Momo hiding under his shirt and warned him to keep it hidden, thinking it was a monkey. Her self-proclaimed boyfriend, Hide, seeing "Kuzon" and On Ji together, stopped by and made it clear to Aang that she was his "girlfriend" by starting a brief confrontation, during which On Ji rolled her eyes, hinting that she was not very affectionate toward Hide. Later, in band class where she played the erhu, Aang tried to play the tsungi horn and got chided by the music teacher for dancing in place while playing. Nevertheless, On Ji was amused by the dance steps and was even offered to be shown some dance moves by Aang.
On Ji was later invited to Aang's dance party. To get people to start dancing, Aang picked On Ji out of the crowd. She blushed, shyly took his hand, and started dancing with him. After this, the rest of the group started dancing around to the music. Later, Aang and Katara began dancing together, and On Ji and the rest of the class began watching in awe as they performed many advanced moves. However, the party was soon broken up, as the headmaster, having been informed by Hide about the party, came into the cave searching for Aang. On Ji, along with the other students, put on a headband to help Aang and Team Avatar escape.
If On Ji considered herself Hide's girlfriend, it was not apparent by her reactions when he was around her. In fact, she was never seen smiling when he was with her and once rolled her eyes when he proclaimed her his girlfriend, thus making their 'relationship' questionable.
On Ji's hair style was the same as Chihiro's/Sen's from Miyazaki's Spirited Away. The creators of Avatar have said before that they are fans of his work.
On Ji was named after Angie, a member of the staff.
On Ji (spelled Onji, 音字) is also a Japanese term; in the context of Japanese poetry, it refers to the phonetic units counted in haiku and other such poetic forms.
On Ji was one of the few Fire Nation natives who did not have the standard black hair and amber eyes.