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
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. 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.

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
Later, he is resurrected by Kabuto, along with the other deceased jinchūriki through the Summoning: Impure World Resurrection, to fight in the Fourth Shinobi World War.

As day breaks, it is revealed that Tobi had turned Rōshi and the other revived jinchūriki into his own customised Six Paths of Pain with each having the Sharingan in their right eye socket and the Rinnegan in the other, similar to Tobi.

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.