Saiken

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

Background
Saiken first came into being in the waning days of the Sage of the Six Paths, who used his Creation of All Things ability to separate the Ten-Tails' chakra from its body and divide it into nine separate constructs that would come to be known as tailed beasts in order to ensure that it would never resurface after his death. Some time after being created, the Sage sat down with all the young tailed beasts and told them that they would always be together, even when separated, and that one day they would become one entity again with different names and forms than they did. Then, when the time came, they would know what true power is.

Saiken eventually fell into the possession of Kirigakure and was sealed into Utakata. In the anime, it was revealed that at some point before his appearance, Utakata's master, Harusame, tried and failed to remove Saiken from within Utakata, intending to save his student from being a living weapon for his 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 the 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 great 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 can also expel a sticky, slime-like substance to trap its target. It has tremendous durability, able to withstand being thrown a great distance by Kurama.

Tsuchigumo Kinjutsu Arc
In the anime, in an attempt to stop the Fury technique from causing untold destruction, Utakata goes into his full Six-Tails form to shield the blast. Ultimately Utakata is captured by Nagato and the Six-Tails was extracted from his body, being sealed into the Demonic Statue of the Outer Path.

Shinobi World War Arc
After Utakata's full transformation into Six-Tails, he attacks Kakashi and Guy, emitting a corrosive alkali from its mouth, which Kakashi warns Guy not to touch. After Guy dissipates the gas with his Morning Peacock technique, the Six-Tails prepares to attack him again. After Son Gokū is resealed into the Demonic Statue of the Outer Path, Tobi, who is intent on going all-out from this point on, forces remaining jinchūriki to transform into their respective tailed beasts.

Preparing to attack the shinobi, Saiken along with the other four beasts create Tailed Beast Balls. These attacks however, are deflected by Naruto and they are rebuffed by a now fully-transformed Naruto. Wasting no time in their attacks, Saiken attempts to incapacitate Kurama by swallowing its arm rendering it immobile. The beast is however swung into Matatabi, sending them both flying away. With this, the opposing beasts prepare a collaborative Tailed Beast Ball which is met by one from Naruto and Kurama. As Naruto enters a deeper plain of the tailed beasts' consciousness, Saiken introduces itself to the young man. Later as the beast is freed from the chakra receiver it affirms Kurama's belief that Naruto was the person the Sage of the Six Paths had spoken about so long ago before being resealed into the Demonic Statue of the Outer Path.

Ten-Tails Revival Arc
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.

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 proceeded to chain them up and prepared to seal them back into the statue. Ultimately, Saiken, along with the other tailed beasts, are all resealed within the Demonic Statue.

Final Arc
Later, the chakra Saiken transferred to Naruto earlier manifested within Naruto's subconscious along with the other tailed beasts to speak with the young ninja and the spirit of Hagoromo Ōtsutsuki. There, they voiced their shared belief that Naruto is the child of prophecy. Ultimately, Kaguya Ōtsutsuki who had taken control of Madara was defeated by Naruto Uzumaki and Sasuke Uchiha and sealed using the Six Paths — Chibaku Tensei technique. The beasts were all ejected from the statue and then summoned back to the real world by Hagoromo and the previous Kage. After the past Kage return to the after life, the tailed beasts begin discussing their plans for the future now that they are finally free. Sasuke however makes his plans known. Desiring to kill the current Kage and seize control of the tailed beasts for his plans of revolution in the shinobi world, he quickly accomplishes the latter with his Rinnegan. With the tailed beasts subdued by his genjutsu, Sasuke then seals them away within his Chibaku Tensei. After the battle when Sasuke declared Naruto the victor, 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.