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 has two long horns on its head which curve upwards, these horns originate from its forehead and form a crown-like plating with darkened tips. Son also has elongated canine teeth which have dark-coloured tips and spike-like protrusions along the length of its tails. In its mouth, it has a big, round opening, from where it spits lava, and it doesn't seem to have a tongue. 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 to fully transform into Son Gokū and attack Killer B and Gyūki, managing to flip the latter over by its tentacles. Later, Son attempted to swallow Naruto. Naruto tried to free himself from between Son's jaws, but failed.

A chained-up Son asked Naruto, now inside the tailed beast, if he had come to steal its power. It lashed out when Naruto referred to it as the "Four-Tails", demanded that Naruto address it by its proper title. As their conversation continued, Son was shocked to discover Naruto to be an open, honest young man. After Son cursed humans for sealing the tailed beasts away and denying their existence, Naruto shared that he was once treated in a similar manner and that he hated to see someone like Tobi controlling the tailed beasts. After initially laughing at the prospect of Naruto, a jinchūriki, wanting to befriend his tailed beast, Son realised that Naruto's ambition was genuine and told him that there might be a way to stop its rampage if the chains that bound it were undone.

In parting, Son bumped fists with Naruto, now eager to know how to undo the chains. Son imparted that in the past he made it second-nature to never trust jinchūriki; it warned Naruto not to expect it to become his ally. Son proceeded to tell him the location of the chakra receiver embedded in its body. Naruto used shadow clones to force Son to vomit him up. A Naruto clone who stayed inside the tailed beast's mouth went into Sage Mode and push the receiver from the inside while he himself pulled it from the outside. Although Naruto succeeded, he quickly realised that Son was still bound. Son explained that the Demonic Statue of the Outer Path still held its chakra and that Naruto had only freed it from Tobi's control. 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 its 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 Son, wondering whether or not they could have traversed a path more similar to Naruto and Kurama's. Son told Rōshi that if he wasn't so stubborn and hadn't forgotten its name all those years ago, it might have been possible. As the beast was resealed into the Demonic Statue of the Outer Path, it parted on good terms with Rōshi, who told it that he had never forgotten its name, but rather was too proud to address him properly. He then wondered if calling the beast "Son" would be appropriate.

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 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 is also 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.
 * The fur on its chin and on the top of its back resemble Rōshi's beard and hairstyle.

Quotes

 * (Introducing itself to Naruto)