Spirit of the Tree of Ages

"I suppose this is one path as well. Perhaps this is all for the best: To remain unchanged."

Spirit of the Tree of Ages is the very vital essence/spirit/soul/core of the Sacred Tree of Ages itself from both timelines and those unseen of either the distant past or future. It had remained dormant for the entire anime of InuYasha.

History
During the events from when Kikyō had pinned Inuyasha on its trunk, the greatly strong spiritual energy within the sealing arrow tip was infused with the uppermost-level priestess's own vital essence. This said essence was absorbed by the Sacred Tree, enabling it to take on the appearance and voice of the late Kikyō whenever it desires to commune directly to any individual.

As Kirinmaru's older sister Zero found Towa and Setsuna, and after she ordered Homura to burn the forest, Jaken tells a sleeping Rin that she already found them and saw the scale is growing bigger. The spirit tells him that Towa has travel through time to the other era and use the Dream Butterfly chrysalis on Setsuna's neck to slow down the curse.

While watching over Rin within its very core, it detected the arrival of the Half-Demon Princesses and requested that the cousins slay the Eastern Beast King Kirinmaru to save all of time and space themselves from "the Degenerate Age."

Many months later, Moroha tries and fails to get it emerge again as the Half-Demon Princesses had just encountered and fought Kirinmaru on its behalf.

Setsuna's upgraded and supremely powerful naginata, now called the Yukari no Tachikiri, began reacting to the fatal curse, as well her, that Zero had placed upon Towa and Setsuna's young human mother, Rin, as leverage over their mighty father Sesshōmaru. To this end, the essence of the Sacred Tree emerges before the Half-Demon Princesses once they enter within, who informs them of Rin's condition and that Setsuna's own curse from the Dream Butterfly was all for gradually reducing Rin's own curse and preserving her young life by stopping her aging process.

Many months later, it reappears before a sorrow-stricken Rin, Jaken and Setsuna and informs them of how to help Towa and Sesshomaru as Akuru appears to lend his own assistance.

Personality
Being infinitely wise and knowledgeable, it also seems to have acquired a bit of Kikyō's personality as it has taken on her form to commune with Sesshōmaru, and his twin daughters and half-niece.

Physical Description
It has no known form but has since borrowed the form and voice of the late priestess Kikyō.

Trivia

 * In the Avatar franchise, there is the ancient Tree of Time of the Spirit World, where the very first Avatar, Wan, had permanently merged with the mighty Spirit of Light and Peace, Raava, who was then transformed into the Avatar Spirit. Avatar Wan had then imprisoned the Spirit of Darkness and Chaos, Vaatu, inside its hollow for the next ten millennium. The Tree of Time remembers everything in existence.
 * Despite its wisdom, the spirit seems to get some facts mixed up, or possibly distorts the truth. Kirinmaru at present is content with his territory, with Zero being the only real threat.
 * It seems the spirit made a pact with Sesshōmaru to watch over Rin; given he can freely enter the dimension within the Tree of Ages. It also gave Jaken the instructions on how to prevent Rin's third and final premature demise via Zero's silver-scale curse by connecting Rin's youngest twin daughter to Rin via a Dream Butterfly.

Media Appearances

 * Episode 4 (Hanyō no Yashahime)
 * Episode 8 (Hanyō no Yashahime)
 * Episode 27 (Hanyō no Yashahime)
 * Episode 28 (Hanyō no Yashahime)
 * Episode 37 (Hanyō no Yashahime)
 * Episode 41 (Hanyō no Yashahime)