Saya

is the sheath spirit of the demon sword Sō'unga. He is the spirit who was supposed to guard Sō'unga in order to stop the sword from being drawn.

Inu no Taishō's will
After the death of Inuyasha's father, Saya Myōga and Tōtōsai were following their master's last will. Tessaiga was sealed inside Inu no Taishō's tomb with the Black Pearl, and Tōtōsai made sure Sesshōmaru would receive Tenseiga (He did not give Tenseiga to Sesshōmaru in person as he feared would have been killed). They were unsure about what to do with Sō'unga though. They couldn't give it to Inuyasha, as he was merely a baby at the time and neither Sesshōmaru, who desired power and would kill them for being given Tenseiga. Saya suggested he'd hold the sword off quietly for 700 years, and placed it in the Bone-Eater's Well.

During the story
About 200 years later, the sword ended up in Kagome's family's possession. In the 20th century, the seal broke. Therefore, Saya tried finding a strong demon, in hopes that they would be able to control Sō’unga before it could unleash chaos in the modern world. He eventually found Inuyasha and Kagome. Inuyasha did not know who Saya was when he arrived (Inuyasha was an infant the last time he met Saya). Say showed the two of them a vision of what Sō'unga was capable of if it was left unchecked. The world was devastated and littered with bones. That motivated Inuyasha to try and subdue the sword.

When Inuyasha grasped Sō’unga, he was taken over, but regained control for a short period of time. To prevent it from harming anyone in the future, Inuyasha went into the past. Kagome was anger that Inuyasha did not accept her help. She asked Saya if he had the power to suppress So'unga. Saya was unsure that he could.

Saya and Kagome retuned to the Feudal era to stop Sō'unga. When they were greeted by Kagome's friends, Myōga accused Saya of slacking from his duties (as So'unga was released after only 200 years).

Inuyasha left his friends to find Sō'unga on his own after he was freed from its control. The others waited for Kagome to regain consciousness after she saved Inuyasha from the sword. As they waited, they learned about the history of Sō'unga. Saya explained to Kagome and the others that the only way to defeat So'ing was using Tessaiga and Tenseiga together. Everyone else knew that would have been impossible (as Inuyasha and Sesshōmaru despised each other). Kagome asked Saya if he could seal the sword again, but Saya made a lot of excuses as to why he could not.

After Sō'unga was being wielded by Takemaru, he erected a giant fortress that was visible from a distance. Everyone was headed there to defeat Sō'unga. Before the battle against Sō'unga, Saya inquired to Kagome about why she was going to fight when she had no stake in the fight. He initially believed that it was because Kagome loved to fight, but Kagome stated that was doing this because she wanted to be with Inuyasha. Saya, along with Myōga and Tōtōsai did not understand her reasoning.

Saya was taken with Kagome and Rin by an ogre (Kagome had Sō'unga's sheath with her when she was capture). All three were taken to Takemaru's chambers. They soon escaped when Sesshōmaru arrived to save Rin.

When Sō'unga was able to move by himself, he opened a path to the netherworld. Saya created a protective barrier for the humans (They would have died if the path to the netherworld was opened).

Afterwords
Saya apparently knew of his master's wish for Inuyasha and Sesshōmaru to seal Sō'unga into the netherworld by working together, but he did not reveal that. He initially stated that he withheld that information because he was thinking about what was best, but Myōga knew that Saya actually forgot to tell anyone.

He was last seen in Tōtōsai's forge where Tōtōsai was forging a new sword for Saya's sheath. It ended up as a small dagger (mush to Saya's distress).

Personality
He is a slacker and is said to be unreliable since he didn't hold Sō'unga as long as he was supposed to. Saya said in a meeting with Myōga and Tōtōsai after the death of the Inu no Taishō that he would be able to seal the sword for about 700 years. However, he was only able to do so for 200 years. He often tells people that bad things will happen after they already have, cementing the belief that he cannot be trusted with anything important.

Despite his flaws, Saya is still a very knowledgeable person. He was aware of things that not even Myōga or Tōtōsai knew of (such as the existence of the Bone-Eater's Well).

Powers & Abilities

 * Invisibility: As a spirit, Say had the ability to make himself invisible.
 * Barrier: Saya had the ability to put up a barrier to protect the humans from the effects of having the gate to the netherworld opened by Sō'unga.

Saya