|Ghostly girl in the Netherworld Tower|
Final boss, formerly the mistress of old Saigyouji clan, currently the ghost princess.
Her main ability is manipulation of death.
Long ago, there was a divinely talented poet in Gensokyo. He loved nature and spent most of his time traveling. When he became aware that the hour of his death drew near, he chose to begin his eternal rest under the most beautiful cherry tree he could find..
That cherry tree has grown even more beautiful since then, charming many people into resting in peace beneath its blossoms. The tree with the mortuary charm developed the powers of a youkai after a while.
A thousand years have passed.
In the garden of the Saigyouji clan, there exists a youkai cherry blossom with a story behind it; it is called the Saigyou Ayakashi. For as long as Yuyuko had been there, this cherry tree has never bloomed, even in spring.
One day, while browsing through the library as usual, Yuyuko found an old record. It looked so old that it was practically ageless; within it, the following was written...
- "The girl who bears witness to death, at the time of the Saigyou Ayakashi's full bloom, shall be bound by mortality. So that her soul may rest in peace within the Hakugyokurou, I will seal the cherry blossoms, and make it the barrier. My wish... is that she will never again reincarnate, and never again suffer pain..."
Yuyuko had read that breaking the seal on the Saigyou Ayakashi, making it bloom fully, would cause someone to be resurrected; she started gathering spring's essence out of curiosity.
Basically, almost everything in the netherworld exists as ghosts with no physical body. It is for that reason that she wondered about the corpse lying under the Saigyou Ayakashi with great interest, going as far as to consider breaking the seal. Her ability only enables her to bring death. Bringing life to something already dead would be a first.
Thanks to an all-out effort put forth by Youmu, the gardener, every cherry blossom bloomed perfectly; every cherry blossom except for the Saigyou Ayakashi, which still bordered on blooming. By that point, Gensokyo's spring had almost been completely drained.
No one would have imagined that a living human, bearing a tiny drop of spring, would visit.
A battle for that last remaining bit of spring ensued...
Perhaps it was because of her carefree afterlife, or maybe the record had aged beyond recognition, but Yuyuko ultimately failed to realize that the girl who was written about in the record... was herself.
In the beginning, Yuyuko was only able to control some dead spirits. Unconsciously, that power grew stronger, eventually becoming the ability to invoke death. Terrified by her own ability, she commited suicide.
After becoming a ghost, she thoroughly forgot everything about the time she was alive, and started to enjoy being able to invoke death.
The reason Yuyuko can neither reincarnate nor vanish is because of the seal on the Saigyou Ayakashi. Once the boundary is breached, the time that was frozen will continue to flow, resulting in her death again. Reviving the corpse will destroy Yuyuko's ghost, so it's no wonder that the near-success of the revival ended in inevitable failure.
Yuyuko is only able to bring death after all.
The only person who knew the whole story, Youki Konpaku, grew slowly older due to the nature of half-humans, and disappeared with the belief that Youmu could inherit the truth about Yuyuko.
For all time, Yuyuko will remain as the princess of Netherworld and mistress of long-dead Saigyouji clan.
- She shall never bear witness to the blooming of the Saigyou Ayakashi.