Son Gokū

, more commonly known as the, is one of the nine tailed beasts. It was last sealed within Rōshi of Iwagakure.

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 had 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. Son has a high amount of respect for those who respect the tailed beasts.

Appearance
Son Gokū is a red-furred and green-skinned monkey, with a body-build of a gorilla. It has eyes with yellow irides and white pupils, spike-like protrusions along the length of its tails, elongated blunt fangs, and two long horns curving upwards on its forehead like a crown (both its fangs and horns are dark-tipped). In its mouth, Son 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 leaner with smaller fangs.

Abilities
As a tailed beast, Son Gokū possesses an enormous supply of chakra and is able to create the Tailed Beast Ball. Son also has the ability to simultaneously combine its earth and fire natures, to spit lava from its mouth. In addition, although being never used in the storyline, it seems to be capable of breathing a stream of green coloured fire from its mouth. Son has a great deal of physical strength, being able to lift and toss Gyūki with ease. Son also seems knowledgeable in taijutsu, likewise showing deceptive nimbleness of its size, utilising powerful and well-timed kicks against its opponents.

Itachi Pursuit Arc
After an unseen and difficult battle with Kisame Hoshigaki, Rōshi was defeated and captured by the Akatsuki member. The Four-Tails 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 the Four-Tails 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 the beasts jaws, but failed.

The chained-up Four-Tails 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; Son Gokū. 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 it properly. He then wondered if calling the beast "Son" would be appropriate.

Ten-Tails Revival Arc
Son Gokū appeared alongside the other tailed beasts with the exception of Gyūki and Shukaku when the Ten-Tails looked down at Naruto. The image of the tailed beasts, along with a silhouette of the Sage of the Six Paths, drove the Ten-Tails to a fit as it evolved further. Later, Son Gokū was extracted from Obito along with the other tailed beasts by the combined efforts of the Allied Shinobi Forces. Upon being freed, Son Gokū noted that Naruto had truly kept his promise and saved them. Upon learning that Gaara — who noted that he considered Naruto his first friend — intended to seal Madara with Shukaku's assistance, Son declared that the tailed beast would also assist with this to repay Naruto.

Return of Madara Arc
As Naruto in Tailed Beast Mode pinned Madara down with a tail, Son and the other tailed beasts joined in and did the same to increase the damage, but the Uchiha survived their attacks and retreated to a safe distance. After he received his own right Rinnegan eye from a White Zetsu clone, Madara quickly summoned the Demonic Statue of the Outer Path from Obito's body, and used the Rinnegan to repel the tailed beasts. With the tailed beasts stunned, Madara then proceeded to chain them up and prepared to seal them back into the statue.

Video Games
Although Son Gokū itself is not playable, Rōshi takes on its form as his awakening in Naruto Shippūden: Ultimate Ninja Storm 3.

Trivia

 * 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)