Onji came to know of the destruction of the Asuka Village and discovered the amnesiac Goh in the forest. He sent him a letter attached to an arrow instructing him to find his "father" in Utakata Castle. After Goh comes back without much clues, Onji sends him another letter apologizing for the misunderstanding, explaining that he meant a "fatherly figure", as Ichijo had been to Goh in the past. In the same letter, Onji informs him that his memories and soul have somehow been stolen and placed within eight mystical stones, the "Soul Fragments", which were scattered during the initial attack on the Asuka ninja's village, and then further scattered by people who have located and claimed the stones.
For the duration of the game Onji guides and advices Goh in his quest to retrieve the Soul Fragments, and more mysteriously, seems to relate closely to Goh's actions and the fragments contents.
Ichijo sends Goh a letter instructing him to retrieve a Soul Fragment hidden in the decorative roof of Utakata's Castle. However, before he arrives to his destination he is confronted direcly by Gamuran, who removes his soul from his body and transfers into the body of a bear, then shorly captured by Ichijo's soldiers and brought to the castle.
Goh barely escapes, and once back into the hut Onji reveals himself as the cat that's been living in the abandoned hut. Long before the events of the first game, Onji abandoned his family to live the rest of his life as a yamabushi. During his pilgrimage he encountered Gamuran, shortly after the sorcerer had destroyed the Asuka Ninja Village, and had his soul transferred into the body of a cat, doomed to gradually lose his human mind. While he knows it was Goh who freed Gamuran, he doesn't blame him for his current state, instead he considers it punishment for abandoning his family. To avoid the same fate, Onji explains the origin of Gamuran and the Asuka Ninja, and instructs Goh to find his body in the forest before it's too late.
After retrieving his body, and shorly before Gamuran reveals himself to Utakata, Goh encounters Onji in the hut once again. He tries to talk to him, but Onji hisses at him and runs away. Onji had lost his mind.
Goh prepares of his final encounter with Gamuran, joined by Kinu and Zaji. During the battle Kinu strikes the sorcerer with the sacred sword hidden in the Asuka Shrine, the only weapon capable of killing him, but is incapacitated by him. Before Gamuran can deal the fatal strike though Onji pounces, distracting the sorcerer and giving Kinu time to give the sacred sword to Goh, at the cost of Gamuran severely injuring him.
In the end Goh proves victorious, killing the ancient sorcerer and burning the Asuka Shrine to get rid of any trace of him. Kinu comes to his side with Onji in her arms, prompting Goh to ask her if he's alive. Kinu sadly shakes her head...
- Serving as something of a plot hole, it's unknown how Onji came to send Goh letters by arrow if he was trapped in the body of a cat for the duration of the game.
- In Shinobido 2: Revenge of Zen a cat very similar to Onji can be seen in some promotional artwork and a few times in the game.