Oneko was the identity assumed by a human woman who lived in Japan in the late 16th and early 17th centuries. The name "Oneko" means "humble cat" in Japanese.
The woman who became known as Oneko was the granddaughter of the leader of a ninja clan from Koga province. Her clan was opposed to Tokugawa Ieyasu. In 1597, the woman's aunt Kiku accompanied a concubine named Oneko on her journey to Fushimi Castle, the home of Torii Mototada, Tokugawa's retainer, with the intention of spying in Mototada's house. Oneko became sick on the road with a lung illness and was taken to a nearby farm. After Oneko died, the members of her escort were killed and the kunoichi (female agent) was substituted for her.
Three years later, in 1600, "Oneko" was pregnant with Mototada's child. Fushimi Castle was soon to be attacked by the army of Ishida Mitsunari. The woman's grandfather sent a party of ninjas to rescue her and recover the information she had gathered. Hikaru Sulu arrived in the middle of the battle, having been sent back in time by the omnipotent being Weyland. Playing the role of the faithful concubine, Oneko helped Sulu defeat the ninjas. He accompanied her back to Fushimi Castle, where he swore allegiance to Mototada.
Some days later, Mototada ordered Sulu to escort Oneko to her uncle's house in Edo. On the journey Oneko and Sulu developed romantic feelings for each other, especially after Sulu saved her from drowning in the Okitsu River. While staying at the inn Minaguchiya, they visited the Seikenji monastery, where Sulu carved their names into a stone in the temple garden. That night, the ninjas attacked the inn, accidentally killing Kiku but recovering Oneko. Sulu confronted the ninjas on Miko beach, where Oneko revealed herself as one of them.
They rode to a farm belonging to Oneko's clan, where she explained her history to Sulu. Oneko believed Sulu to be a god or a tengu (a mountain goblin), and convinced her grandfather of this, but her uncle did not believe her and demanded information from Sulu about Mototada's battle plans. Sulu refused to answer. The ninja camp was attacked by warriors led by Watanenabe Sadayo, Mototada's sword master, who rescued Sulu. The next day they attacked the compound again, and Sulu rescued Oneko from the battle.
On the seashore, Oneko pleaded with Sulu to take her into the mountains and live there with her as a farmer, but Sulu was unwilling to live in hiding and without honor. He brought her to Mototada's house in Edo, where they were relieved to discover that her supposed uncle (actually the real Oneko's uncle) had left the city. Sadayo had killed all the ninjas, leaving Oneko's identity safe. Sulu then departed for Fushimi Castle, leaving Oneko behind.
After his return to the 23rd century Sulu returned to Japan, where he and Dr. Yae Takenada, an archeologist, unearthed the stone engraved with Sulu and Oneko's names in the temple garden. (TOS novel: Home Is the Hunter)