Rōshi

was the jinchūriki of the Four-Tails that hailed from Iwagakure.

Background
Rōshi became the jinchūriki of the Four-Tails 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.

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, and brown armour. He also had a bag tied on his waist, most likely containing ninja tools.

Abilities
He has been said to be strong as Kisame, a powerful Akatsuki member himself, stated that he was not one to play around with as he was tough, a testament to his skills. 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.

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, unleash the power of their 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.

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 headset seems to have disappeared in the anime.