Shiranui is a female Ho-Oh in the backstory of the Suocéverse and the Protector Ho-Oh previous to the role passing to her second son, Ho-Oh.
Shiranui was born at least 600 years before the events of Generation 1 and was promoted to the role of Protector at some point in the last 400 years, stationed in the region of Ransei.
At some point right before the events of the Suocéverse version of Pokémon Conquest, Shiranui decided to send her two children away to the region of Krasnoska as the power of a Ho-Oh was needed there.
Later, during the events of Conquest, Shiranui herself would be killed during the emergence of the Ghost Lords. With her death, Ho-Oh had to be promoted to the role of Protector at an early age.
End spoilers.
Shiranui draws her backstory from the backstory of the similar character from WAAPT.