Rōshi

was the jinchūriki of Son Gokū, the Four-Tails, that hailed from Iwagakure. The Four-Tails has been captured and sealed by Akatsuki, causing his death when the beast was extracted. However, upon his resurrection, the Four-Tails appears to have been resealed within Rōshi.

Background
Rōshi became the jinchūriki of the Four-Tails over forty years before the Fourth Shinobi World War at a very young age. He left his home town of Iwagakure at some point to better understand and control the power of the beast within him.

Personality
Son mentioned that Rōshi was just as hard-headed as Ōnoki, the Third Tsuchikage, suggesting the two had been acquainted. Despite this, upon observing Naruto Uzumaki's battle against Tobi and his Six Paths of Pain, as well as his knowledge of the tailed beasts final meeting with the Sage of the Six Paths, he came to regrettably believe that all the jinchūriki could have walked down the right path if they were more like the young man. Also despite being Son's jinchūriki for over forty years, he never referred to the beast by its true name, stating that he had not become soft-hearted enough to do such a thing. Before Son was resealed into the Demonic Statue of the Outer Path, he revealed to the beast that he never forgot its name, making peace with the giant monkey.

Appearance
Rōshi was shown wearing a large headpiece consisting of a three pointed crown-like ridge, bearing both his Iwagakure forehead protector and ring, and a prominent black piece of armour running across his cheeks and the bridge of his nose.

Despite his advanced age, his hair was solid red. His hair and beard style were exactly the same as that of the Four-Tails inside him. He dressed himself in a long-sleeved purple shirt and pants, with a mesh-armour shirt underneath, and brown armour. He also had a bag tied on his waist, most likely containing ninja tools.

Abilities
He has been noted to be a very powerful shinobi. Kisame, a powerful Akatsuki member himself, stated that he was not one to play around with as he was a tough one to capture. He was also seen using a curved kunai with which it can be assumed that he had proficiency wielding.

Nature Transformation
By borrowing the Four-Tails' power, Rōshi was able to use its Lava Release ninjutsu, a combination of fire and earth-based chakra, to create a torrent of lava that can melt almost anything. In the anime, Tobi referred to him as, further indicating his prowess with it. He could create a giant geyser of lava as seen in the anime or spit multiple balls of lava at the enemy. He could also create an armour made of lava that not only has defensive purposes but adds lava to his taijutsu attacks increasing their potential damage.

Jinchūriki Transformations
As the Four-Tails' jinchūriki, Rōshi is granted a significant amount of chakra and stamina from the beast. The full extent of his control is unknown, but he could form at least one of the beast's tails upon his resurrection, further enhancing his power and strength. After being immobilised by B's clone, he transforms into his Version 2 form, in this form he was able to create an enormous torrent of lava, scorching the Eight-Tails. When Rōshi attacked Naruto and Killer B, Tobi forced him to transform into his full Four-Tails form. In this form, he displayed tremendous strength, being able to toss Killer B in his full Eight-Tails form a considerable distance away.

Itachi Pursuit Arc
After an unseen and difficult battle with Kisame Hoshigaki of Akatsuki, Rōshi was somehow defeated and captured by Kisame. He later died as the result of having the Four-Tails extracted from his body.

Shinobi World War Arc
In preparation for the Fourth Shinobi World War, Rōshi is resurrected by Kabuto Yakushi to fight against the Allied Shinobi Forces, before being mobilised alongside the other deceased jinchūriki.

As dawn breaks on the second day, it is revealed that Tobi had turned the revived jinchūriki into his own customised Six Paths of Pain, with each having a Rinnegan and Sharingan in place of their left and right eyes respectively. Each jinchūriki also had their particular tailed beast resealed within them and a chakra receiver embedded into their bodies.

Rōshi is later seen travelling together with Tobi, in pursuit of Naruto and Killer B. Encountering their targets, the revived jinchūriki were sent into battle with Rōshi leading the offensive, using his Lava Release: Scorching Stream Rock Technique to pressure the enemy. When this initial assault fails, he, like the others, unleashes the power of his respective tailed beast by materialising the first of its tails.

Whilst Naruto and B converse, Rōshi exploits this opportunity to attack the former directly, after covering his body in lava. Despite evading the strike, Naruto's face is still scorched by the heat but when Rōshi attacks again, Naruto endures the blow in an attempt to destroy the chakra receiver embedded in his chest. However, this is thwarted by Han. Incapacitated in the wake of the Eight-Tails' devastating attack, after B's recent transformation, Rōshi is then restrained by the latter's sealing technique. However, before the technique's completion, he manages to escape by adopting his Version 2 form, where he then proceeds to burn the Eight-Tails using his Flower–Fruit Mountain technique.

Regrouping with the other revived jinchūriki, after Kakashi Hatake and Might Guy's intervention, Rōshi charges past the pair towards Naruto and B. When he clashes with the former, Tobi forcefully transforms him into his full Four-Tails form, where he then proceeds to assault the Eight-Tails. Grabbing one of the beast's tails, Rōshi hurls the two jinchūriki upwards and, after dislodging Naruto in the process, prepared to swallow him. However, despite Naruto's efforts to lodge himself between the beast's jaws and B's attempts to restrain the ape, the former is ultimately swallowed whole.

While Naruto converses subconsciously with Son Gokū, Rōshi resists B's efforts to free his captive comrade, only to be forced to eject Naruto when he multiplies. He then tries to impede Naruto from removing the chakra receiver embedded in the beast's chest, but fails to do so thanks to a coordinated strike, leaving Rōshi's incapacitated body behind as Son Gokū is resealed into the Demonic Statue of the Outer Path.

During the clash of Naruto's and Kurama's Tailed Beast Ball with the other tailed beasts' collaborative Tailed Beast Ball, unconscious Rōshi's body is protected by Gyūki from the consequences of the explosion. After the clash Naruto meets with the other tailed beasts and their jinchūriki, it is revealed that before Son was sealed into the Demonic Statue of the Outer Path, Rōshi tried to make peace with Son much to the giant monkey's shock. As they part, he tells the beast that he had not forgotten its real name, but was not soft-hearted enough to have called the beast by it. As they fade away, a smiling Rōshi asks the beast if calling it simply "Son" would be all-right.

Trivia

 * means "old age" or "elderly". means "purple". It is possible Rōshi was named after the Dragon Ball character Master Rōshi, considering that the Four-Tails' design was also inspired by Dragon Ball.
 * In order to prevent the jinchūriki from turning traitor, it is tradition for the host to be selected from the family of the village's own Kage. It might be that Rōshi somehow was connected with one of the Tsuchikage.
 * In the manga Rōshi's headpiece is depicted as having a golden ring on the left side. The golden ring on Rōshi's headpiece was not present in the anime. This was later added as of episode 256.