She appears as a slender and well-endowed and her long hair is let down.
Kirio comes across as a cheery individual, but has been known to lash out at others, striking Shinji Hirako while telling him to "feel pain".
Kirio is known to have lived at least 3,000 years, being one of only only two people alive who attended Shigekuni Yamamoto's birth with the other being Yachiru Unohana, Kirio's best friend. Then, she took in Shigekuni as a guardian, making sure Shigekuni would master Kendo and become the first person to master the Bankai of the Zanpakuto: Ryujin Jakka as requested by Osho.
2,000 years ago, Shigekuni founded the Shinigami Academy, and quickly became popular. Kirio formed the first army of Shinigami from this and using these Shinigami, began building a city-wide military base which Yachiru named "Seireitei", after an ancient legend of a giant city spanning as far as the eye could see. Over the next thousand years, this city started to form shape.
Around 1,000 years ago, Shigekuni was picked to become the leader of a new army: The Gotei 13. Kirio became of the first captains, alongside Shigekuni's most trusted friends: Shunsui Sōzōsa Kyōraku no Jirō, Jūshirō Ukitake, Ginrei Kuchiki, Oetsu Nimaiya and Yachiru Unohana. From there, the army grew into a colassal scale, eventually becoming the guardians of Soul Society.
At some time before 1891, Kirio took an interest in research, and made the Twelth Division the scientific research division of the Gotei 13. She herself experimented with the Soul and how an artificial version could be created from the theory that one could inject foreign reiatsu into one's body, and then for the individual to turn that power into their own. She even incorporated this theory into her cooking, something which made her famous.
In 1891, Kirio was contacted by the Spirit King. The King requested that she become part of the newly found group, known as the Royal Guard. She accepted and on the day she was promoted, the entirety of Seireitei saw her departure. From then, she started taking a secretive duty, preparing for the Special War Potentials.
By the 1980s, Kirio was given quite a mysterious duty: Live in the World of the Living while under disguise. Using her own Gikon technology, Kirio became a typical human, getting a job and a house. During this time, she met a person named Oscar Matthews, who could sense that she had spiritual power and for once, she had someone she could talk to about being different to anyone else on Earth. They fell in love, and in 1984 had a child: Roydon Matthews.
During his childhood, Roydon was sensed to have Spiritual Power in 1985, and Kirio was told to report back to the Soul Palace due to questioning involving Fullbringers. Confused about Oscar, she went back, and revealed there that the Shinigami and Fullbringers are actually arch-enemies and so Kirio must stay in the Soul Palace. Allowed one last time to say farewell to her family, Kirio went back to the World of the Living alongside Senjumaru Shutara. Arriving at Kirio's house, she found nothing but the burning wreckage that she once lived in. Going into the wreckage, she found Oscar, barely alive.
Oscar whispered how a group of Shinigami, but all clothed differently, wanted Kirio and how they mentioned something about "The Kokumotsu Kuchiku". He then handed Roydon to Kirio and Shutara put him into a bubble. Oscar died, as the house's supports started to break. Kirio and Shutara managed to get out, and left the fire to burn as they opened a portal back to the Soul Palace. Shutara told Kirio that she knew exactly who Roydon should be with, and took him as Kirio waved farewell to the child.