Son Gokū

, more commonly known as the, is a tailed beast sealed within Rōshi of Iwagakure. It was captured by Kisame Hoshigaki, then extracted and sealed by Akatsuki. Son was later resealed into a reincarnated Rōshi during the Fourth Shinobi World War.

Background
Son Gokū 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 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 as well as forms than they did then when the time came for them to understand what true power is.

Son Gokū eventually fell into the possession of Iwagakure and was sealed into Rōshi over forty years before the Fourth Shinobi World War, who later left Iwagakure in a journey to gain a better understanding and control of the tailed beast.

Personality
Son Gokū is a very prideful tailed beast, as it detests being called by the name "Four-Tails" and boasts about its lineage. It uses when referring to itself.

It also hated humans, due to the fact that humans have enslaved it, and believes that apes are more intelligent than humans. However, when talking about the Sage of the Six Paths, it showed him great respect. It also seems to have taken a liking to Naruto Uzumaki, as it was moved by his actions for truly trying to save it from being controlled by Tobi and even gave him some of its chakra before being sealed away.

Appearance
Son Gokū has yellow irides, white pupils, red fur and green skin. He primarily resembles a monkey, but has the build of a gorilla. It also has horns on its forehead, similar to the Third Tsuchikage's yellow jacket, as well as elongated canine teeth and spike-like protrusions on the four of its tails. Its long canines appear to be broken and are darker at the end. In its mouth it has a big, round opening, from where it spits lava, and it doesn't seem to have a tongue. The fur on its chin and on the top of its back resemble Rōshi's beard and hairstyle. During the last remaining days of the Sage of the Six Paths, Son was much smaller and had smaller teeth.

Abilities
Son Gokū has the ability to simultaneously use the earth and fire natures, to spit lava from its mouth; something that hasn't been seen outside the realm of kekkei genkai. As it's jinchūriki, Rōshi could borrow its ability to create and use lava. As a tailed beast, Son is able to create a Tailed Beast Ball. Son Gokū is very strong as it was able to lift and toss Gyūki.

Itachi Pursuit Arc
After an unseen and difficult battle with Kisame, Rōshi was defeated and captured by the Akatsuki member. Son was later extracted and sealed, leading to Rōshi's death.

Shinobi World War Arc
During the Fourth Shinobi World War, Tobi forced Rōshi into his full tailed beast transformation. With Son Gokū on the battlefield, the great monkey immediately attacks Gyūki and managed to grab it by its tentacles and flip the beast over. It later attempts to seemingly swallow Naruto, but as the young jinchūriki attempted to free himself from between the Son's jaws, Son opened its mouth even wider and swallowed him.

Now inside the beast, a chained up Son asked Naruto if he had come to steal its power as well, and lashed out at the young man when he referred to it as the "Four-Tails" telling him to address him by his proper title and with due respect. As their conversation continued, Son found Naruto to be an open, honest, person which shocked him somewhat. After cursing that all humans did was seal the tailed beast away and denied their existence, Naruto shared that he too was treated like that once and hated to see someone who did not care who they were controlling the beasts in this manner. After laughing at the prospect of Naruto wanting to be friends with his tailed beast, Son realised that Naruto was genuine in his feeling and told him that there might be a way to stop him if the chains that bound it were undone.

As an eager Naruto asked how to undo the chain that bound him, Son told the young man beforehand that he had made it a point not to trust jinchūriki and not to expect for it to become his ally unless he trusted him. As Son was forced to vomit Naruto out after the latter used the Multiple Shadow Clone Technique, the beast told Naruto the location of the chakra receiver that was embedded into him, which had since moved because of his transformation. While still being manipulated to attack Naruto, a Naruto clone who had stayed inside the beast's mouth prepared a two-fold attack on the chakra receiver after going into Sage Mode. As the plan succeeded and the receiver was removed from Son's body, Son noted that despite this the chain still bound him. As Naruto argued with the beast over wanting to save it completely and not just free it from Tobi's control, Son in disbelief of the young man's character, offered some of his chakra to Naruto, pleased that he genuinely tried to rescue it. Son was then resealed into the Demonic Statue of the Outer Path, leaving Rōshi's body behind.

In Fū's flashback of the time before Son was sealed it was revealed that Rōshi attempted to make amends with the beast, wondering whether or not they could have walked the same path as Naruto. Son told Rōshi that maybe if he wasn't as stubborn as Ōnoki and had forgotten its name all those years ago, it might had been possible. As the beast is resealed into the Demonic Statue, it parts on good terms with Rōshi who told him that he never forgot his name but was too prideful to do such a thing and then asked whether calling the beast "Son" would be all-right.

Trivia

 * 'Son Gokū' is a Japanese name for 'Sun Wukong', a legendary character from the classic Chinese novel Journey to the West. Kishimoto was mainly inspired to create Son Gokū based on the character with the same name from the manga series Dragon Ball. In addition, its design is based on Goku's Ōzaru form and his inherited Dragon Ball that has four stars in it. Besides the name, Son Gokū is based on several other references to Sun Wukong, which includes:
 * The horns on its forehead are modelled after Sun Wukong's diadem.
 * and are both titles that Sun Wukong bears.
 * The is the cave where Sun Wukong was born on the.
 * When Naruto met Son within Rōshi, it was sealed to a giant stone, which reflects the mountain under which Sun Wukong was sealed.
 * A can be one of two things. It can be the name given to a newly ordained Buddhist monk or nun, or it can be the posthumous name given to deceased people according to the Buddhist faith.

Quotes

 * (Introducing itself to Naruto)