Saiken

, more commonly known as the, is one of the nine tailed beasts. It was last sealed within Utakata of Kirigakure.

Background
Saiken and the tailed beasts first came into being in the waning days of Hagoromo Ōtsutsuki, who used his Creation of All Things Technique to divide the Ten-Tails' chakra into nine separate bodies and giving each a name. Some time after being created, the Sage explained to the young tailed beasts that a link existed between them despite their separation. He also foretold of a day when they would be brought together again - though not as they were - and that at that time someone would emerge to show them what true power is. After Hagoromo's death, the tailed beasts drifted apart, apparently due to Kurama's belief that their number of tails was an indication of their strength. Saiken eventually was captured by Hashirama Senju, and was distributed alongside Isobu to Kirigakure, where it was sealed in Utakata decades later.

In the anime, Harusame tried and failed to remove Saiken from within his student, intending to save his Utakata from being a living weapon for their village. Utakata, however, having not heard what his master had said to him, later came under the assumption after the incident that his master had tried to kill him. This failed removal resulted in Utakata transforming into Saiken and killing Harusame.

Personality
Though much has not been seen of Saiken's personality, the giant slug uses when referring to itself, "ore" being a boastful way of saying "I". It also speaks with a dialect which may possibly be Ōsakaben in origin. In the anime, Saiken also seemed to be kinder and not as self-serving and malicious as most other tailed beasts, as when Utakata asked for its help to save Hotaru it seemed to willingly comply; and later it simply receded back into Utakata without trying to take him over. However it was not above killing for self preservation, as seen when it killed Harusame when he attempted to extract it from its jinchūriki. In the anime, it speaks with a high pitched voice and appears to be very energetic.

Appearance
Saiken resembles a large white, with a slight light blueish tint, bipedal slug with stubby arms and feet. It has two prominent optical tentacles (eyes) and hole-like openings as a mouth. Its entire body is covered in a slimy substance. During the last remaining days of the Sage of Six Paths, Saiken was much smaller than its present-day self, but was still much larger than the Sage.

Abilities
As a tailed beast, Saiken has a massive amount of chakra and can use it to create a Tailed Beast Ball. Saiken has shown the ability to emit corrosive substances that can instantly disintegrate its target on contact in the form of liquid or gas. It has tremendous durability, able to withstand being thrown a great distance by Kurama. Saiken possesses an affinity for Water Release. It can expel a sticky, slime-like substance from its mouth, which is capable of trapping its targets, and in the anime, use Soap Bubble Ninjutsu.

Six-Tails Unleashed
In the anime, in an attempt to stop the Fury technique from causing untold destruction, Utakata went into his full Six-Tails form to shield the blast. Later, Utakata was confronted and captured by Nagato's Six Paths of Pain. Afterwards, Saiken was extracted from Utakata's body, being sealed into the Demonic Statue of the Outer Path, thus killing the jinchūriki in the process.

Fourth Shinobi World War: Climax
After Utakata's full transformation into Six-Tails, he attacks Kakashi and Guy, emitting a corrosive alkali from its mouth, which Kakashi warned Guy not to touch. After Guy dissipated the gas with his Morning Peacock technique, the Six-Tails prepared to attack him again. After Son Gokū was resealed into the Demonic Statue of the Outer Path, Tobi, who was intent on going all-out from that point on, forced Utakata to fully transform into the Six-Tails. The opposing tailed beasts prepared to attack the shinobi, but they were rebuffed by a fully-transformed Naruto.

Wasting no time in their attacks, the Six-Tails incapacitated Naruto and Kurama by swallowing the arm, but the slug was slammed into the Two-Tails, sending them both flying away. The opposing beasts regrouped and then prepared a collaborative Tailed Beast Ball, which was met by one from Naruto and Kurama. As Naruto entered a deeper plain of the tailed beasts' consciousness, Saiken introduced itself to the young shinobi. Later, as Saiken was freed from the chakra receiver, it affirmed Kurama's belief that Naruto was the person the Sage of the Six Paths had spoken about so long ago before being resealed back into the Demonic Statue.

Birth of the Ten-Tails' Jinchūriki
Saiken appeared alongside with the other tailed beasts with the exception of Gyūki and Shukaku when the Ten-Tails looked down at Naruto. The image of the tailed beasts, along with a silhouette of the Sage of the Six Paths, drove the Ten-Tails to a fit as it evolved further. Later on, the Allied Shinobi Forces managed to pull the tailed beasts out of Obito, thereby freeing them, including Saiken. When Madara ran towards the tailed beasts, both Shukaku and Gaara conjured numerous sand bullets and were able to restrain the Uchiha's movements.

Using the opportunity to attack, the tailed beasts each took a swing at their opponent, giving sand-manipulating duo an opening to use their Desert Layered Imperial Funeral Seal, but Madara managed to escape the sand pyramid with his Susanoo. As Naruto in Tailed Beast Mode pinned Madara down with a tail, Saiken and the other tailed beasts joined in and did the same to increase the damage, but the Uchiha survived their attacks and retreated to a safe distance. After he received his own right Rinnegan eye from a White Zetsu clone, Madara quickly summoned the Demonic Statue of the Outer Path from Obito's body, and used the Rinnegan to repel the tailed beasts. With the tailed beasts stunned, Madara then chained them up and sealed them back into the Demonic Statue.

Later, the chakra Saiken transferred to Naruto earlier, manifested within Naruto's subconscious along with the other tailed beasts to speak with the young jinchūriki and the spirit of Hagoromo Ōtsutsuki. There, they all voiced their shared belief to the Sage that Naruto was the child of prophecy.

Kaguya Ōtsutsuki Strikes
After Madara's ambitions were cut short by Black Zetsu, who revealed to be the silent manipulator of shinobi history since it was first created by Kaguya Ōtsutsuki, the black entity used Madara's body as a medium to resurrect its creator. During the battle against the tyrannical princess, Saiken and the other tailed beasts continued to support Naruto by giving him their combined power to overwhelm her. Ultimately, Kaguya was defeated by Naruto and Sasuke, who used the Six Paths — Chibaku Tensei to eject all the tailed beasts from her and seal her away inside a newly-created moon within her own dimension.

After the spirits of Hagoromo and the past Kage summoned both the tailed beasts and Team 7 back to the real world, the tailed beasts began to discuss their plans for the future since they were finally free. Sasuke, however, made his own intentions known: removing as many people as possible who could get in the way of his plans of leading the world through a revolution to do away with current shinobi systems, including killing all the current Kage and tailed beasts. Just as Sasuke swiftly subdued the tailed beasts with his Rinnegan and sealed them within his Chibaku Tensei, Naruto reassured the tailed beasts that he would save them once again. After Naruto was able to defeat and reform Sasuke, the two dispelled the Infinite Tsukuyomi and Sasuke released Saiken, along with the other tailed beasts, from their prisons.

Video Games
Although Saiken itself is not playable, Utakata takes on its form as his awakening in Naruto Shippūden: Ultimate Ninja Storm 3 and Naruto Shippūden: Ultimate Ninja Storm Revolution.

Trivia

 * literally means "rhinoceros dog", apparently a kind of Chinese yōkai.