Koyanskaya is originally introduced as Tamamo Vitch, one of the Tamamo Nine. Shi Huang Di also calls her "Da Ji", a Chinese figure who ruined dynasties.
However, Koyanskaya actually has no real True Name to speak of, and is entirely unrelated to either the Tamamo Nine or Da Ji. Koyanskaya is "the Tunguska event given form", the embodiment of the thoughts of animals persecuted by mankind, and chose to manifest in the Saint Graph of Tamamo no Mae due to their shared attribute of being a "beast that is supposed to live for revenge against humans." She likens herself to "the Tamamo no Mae from another world," considering Tamamo to be her original, but she's not actually Tamamo nor is she part of the Tamamo Nine.
She was given the name Yaskaya by the manifested Heroic Spirit Dobrynya Nikitich, which she later mistakenly remembered as Koyanskaya. This is an allusion to the Shinto goddess Konohanasakuya-hime, to which Koyanskaya has no relation whatsoever.
When she was defeated in her Beast form, she split into two distinct but similar fragments, Koyanskaya of Light and Koyanskaya of Darkness, in a contrast akin to yin and yang.
She has been released as an Assassin class Servant in Fate/Grand Order under the name "Koyanskaya of Light", and shortly afterwards as a Foreigner class Servant under the name "Koyanskaya of Darkness".
