Isobu

, more commonly known as the, is a tailed beast that was formerly sealed in Yagura of Kirigakure, after being sealed in Rin Nohara as part of Kirigakure's plan to attack Konohagakure. It has been captured by Deidara and Tobi, then sealed by Akatsuki. It was later resealed into a reincarnated Yagura during the Fourth Shinobi World War.

Background
Isobu 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.

At some point in the past, Isobu, who was in the possession of Kirigakure, was sealed within a kidnapped Rin Nohara of Konohagakure. They plotted for Rin to be taken back to her village as a Trojan horse, releasing the tailed beast to wreak havoc in Konoha. This failed, however, as Rin was fully aware of their schemes and opted to kill herself at hands of Kakashi before their objectives could come to fruition. Eventually, Isobu was revived and sealed within its next jinchūriki — Yagura, the Fourth Mizukage. Some time later, Yagura died and the beast was by itself when encountered by Tobi and Deidara.

In the anime, Tsunade states that Isobu's disappearance happened after the Third Shinobi World War.

Personality
Isobu speaks in a shy manner, referring to itself as which is usually used by young boys.

Appearance
Isobu primarily resembles a large turtle, but with a crab-like shell, and three shrimp-like tails. Under its shell, it has red, muscle-like tissue. It has a pair of human-like arms and hands, but no hind-legs. Its lower jaw is rather big and has teeth-like horns, together with its big forehead, which also has horns, it somewhat resembles a mouth with big teeth, making it look like the rest of the face is inside the mouth. Its eyes are dark and have red pupils. In the anime, the eyes are red with yellow pupils. Its right eye is constantly closed, indicating some sort of injury, and because of this, it is particularly vulnerable to attacks directed at its right eye. During the last remaining days of the Sage of the Six Paths, Isobu was much smaller than it currently is but was still many times larger than the Sage.

Abilities
As a tailed beast, Isobu possesses an enormous amount of chakra and can perform the Tailed Beast Ball. It also has the ability to create coral, and swim at very high speeds. Tobi suggested that Kisame would have been a better choice for capturing the turtle, given its affinity for water. Isobu has also been shown to be able to roll into a ball and attack its opponents, making full use of its shell and its many protrusions in a manner similar to the Akimichi clan's Spiked Human Bullet Tank. Its tough skin and shell also provides additional defence for Isobu, allowing it to withstand nearly all forms of attack. In the anime, it was revealed that its only known physical weakness it possessed was its eye, as hard as its skin and armoured shell are, it cannot harden its eyes and so seems to keep one closed at all times.

In the anime, Isobu's abilities were greatly elaborated upon: it could create shockwaves to repel attacks and produce a large tidal wave that spread away from it in all directions; it could strike with any of its three spiked tails to break through most defences. With these abilities, it overwhelmed Guren's Crystal Release several times over the course of their battle. Of Isobu's more specialised abilities, it could produce a hallucinogenic mist that exploited the victim's insecurities and forced the victim to face them. It could also shoot powerful chakra-infused water balls. When it ate something, thousands of miniature versions of itself would attack what it had eaten within its stomach, presumably to speed up the digestion process. In addition to its anime appearance, the beast was shown capable of materialising an entrance to a separate dimension in which it could hide its presence, until it was ready to return.

Three-Tails Arc
In the anime, Orochimaru instructed Kabuto Yakushi and Team Guren to take a young boy named Yūkimaru to a lake. After they arrived, Kabuto forced Yūkimaru to release all of his chakra, which caused the Three-Tails to begin to surface from the lake. With Guren's help, the beast was completely revealed, and they attempted to capture it. Their attempts proved insufficient, however, and the beast created a large tidal wave to wash its attackers away.

The tidal wave gained the attention of a nearby group of Konohagakure ninja, who tried to use Four-Corner Sealing Barrier to permanently trap the beast in its own dimension. Just as the sealing began to work, the Konoha ninja were interrupted by Guren, which prompted the beast to attack them all. Yūkimaru was able to calm the beast down temporarily, but, after he became tired, the Three-Tails turned its attention to him. Guren and Naruto Uzumaki came to his defence, but were swallowed in the process. With the help of their allies on the outside, they were able to escape, and the Konoha ninja tried once more to seal the beast.

Just when the sealing was almost complete, Yūkimaru, angered by the apparent death of Guren, empowered the beast to break free of its restraints. The Three-Tails went on a rampage, attacking everyone nearby and crushing Nurari, Kigiri, and Kihō. Its attempts to attack Yūkimaru, however, had no effect. It was only through the use of the Wind Release: Toad Oil Flame Bullet that the beast was driven away. The sealing attempt was left to ANBU members, and the remaining Konoha ninja returned home. Deidara and Tobi of Akatsuki soon found these ANBU members and disposed of them.

The beast's capture took place much earlier in the manga (at the start of the Hidan and Kakuzu Arc), but played out in much the same way as in the anime. Tobi and Deidara confronted the beast, to which it responded by beginning to chase Tobi. With the Three-Tails distracted, Deidara moulded an explosive clay fish in an attempt to capture it. It was unclear if this marked the end of the battle, as the remainder of the battle went unseen. As the two towed the Three-Tails to the Akatsuki's lair after the battle, Tobi gloated at bringing the Three-Tails down with his special technique, but Deidara thought otherwise. It was later sealed in the sealing statue.

Shinobi World War Arc
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 Yagura to fully transform into the Three-Tails.

Preparing to attack the shinobi, the Three-Tails 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, the Three-Tails evades Kurama's attack and then, rolling into a ball, attempts to attack the beast while Saiken had incapacitated it. It is however stopped by Gyūki. Freeing itself from the beast's grasp, it and the other 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, Isobu (who remembers meeting him in the anime) 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.

Video Games
Although Isobu itself is not playable, Yagura takes on its form as his awakening in Naruto Shippūden: Ultimate Ninja Storm 3. Isobu also appears as a boss in Naruto Shippūden: Ninja Destiny 3.

Trivia

 * literally means 'beach stroker', and is also an adaptation of the name of the.
 * Turtles are often used as the base design for sea monsters in Japanese folklore. Isobu's design in particular was conceived as a result of the series creator, Masashi Kishimoto's, interest in monsters, yōkai, and kaijū, mixing together various elements of those types of creatures to create a monster with a turtle's body and a shrimp's tail with a crab-looking figure.
 * Since Rin died without having Isobu extracted, Isobu is the first known tailed beast to have "died" and revived.