Son Gokū

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

Background
Son Gokū and the tailed beasts first came into being in the waning days of Hagoromo Ōtsutsuki, who used his Creation of All Things Technique to divide the Ten-Tails' chakra into nine separate bodies and giving each a name. Some time after being created, the Sage explained to the young tailed beasts that a link existed between them despite their separation. He also foretold of a day when they would be brought together again - though not as they were - and that at that time someone would emerge to show them what true power is. In the anime, before his death, Hagoromo sent Son to live in a temple built for the purpose of protecting it in a desolate rocky region that would later become part of the Land of Earth. Eventually, Son moved on to live in a new location known as the Water Screen Cave, reigning over the monkeys that resided there.

During the First Shinobi World War, Hashirama Senju captured eight of the tailed beasts and sold Son and Kokuō to Iwagakure at the Five Kage Summit, as a sign of friendship and to ensure a balance of power amongst their new villages. The tailed beast would end up being sealed within Rōshi decades later who left his village 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 Six Paths, it showed him great respect, being that also had, apparently, a very deep relationship with him, never forgetting his final words. 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. He also treats his fellow tailed beasts with much respect, as he addresses them by their name, as seen with Kurama. Son also 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 doesn't have a tongue, but has an opening that is shaped like a dome volcano. During the last remaining days of the Sage of 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 can create the Tailed Beast Ball. Son also seems knowledgeable in taijutsu, likewise showing deceptive nimbleness of its size, utilising powerful and well-timed kicks against its opponents. Son has a great deal of physical strength, being able to lift and toss Gyūki with ease.

Nature Transformation
Son has the ability to use Fire Release and Earth Release simultaneously, allowing it to use Lava Release. With it, it can create volcanoes as well as flaming boulders to launch at the enemy, and even armour of molten rock to defend itself. In addition, although being never used in the storyline, it seems to be capable of breathing a stream of green coloured fire.

Itachi Pursuit Mission
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.

Fourth Shinobi World War: Climax
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.

As Son explained to Naruto about the location of the chakra receiver embedded in its body, the beast warned him not to expect it to become his ally, due to its distrust against jinchūriki. 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 jinchūriki's character, offered 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.

Birth of the Ten-Tails' Jinchūriki
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 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 it too along with the other freed tailed beasts would also assist to repay Naruto.

When Madara ran towards the tailed beasts, both Shukaku and Gaara conjured numerous sand bullets and were able to restrain the Uchiha's movements. Using the opportunity to attack, the tailed beasts each took a swing at their opponent, giving sand-manipulating duo an opening to use their Desert Layered Imperial Funeral Seal, but Madara managed to escape the sand pyramid with his Susanoo. 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 chained them up and sealed them back into the Demonic Statue.

Later, the chakra Son transferred to Naruto, earlier manifested within Naruto's subconscious along with the other tailed beasts to speak with the young jinchūriki and the spirit of Hagoromo Ōtsutsuki. There, they all voiced their shared belief to the Sage that Naruto was the child of prophecy. As Naruto fought Madara, Son aided Naruto by lending him its Lava Release chakra to empower his Rasenshuriken, which completely overwhelmed Madara and cut down the tree form of the Ten-Tails.

Kaguya Ōtsutsuki Strikes
After Madara's ambitions were cut short by Black Zetsu, who revealed to be the silent manipulator of shinobi history since it was first created by Kaguya Ōtsutsuki, the black entity used Madara's body as a medium to resurrect its creator. During the battle against the tyrannical princess, Son and the other tailed beasts continued to support Naruto by giving him their combined power to overwhelm her. Ultimately, Kaguya was defeated by Naruto and Sasuke, who used the Six Paths — Chibaku Tensei to eject all the tailed beasts from her and seal her away inside a newly-created moon within her own dimension. Son complimented Kurama for having such a good jinchūriki even though it admitted the boy was a bit naive at times.

After the spirits of Hagoromo and the past Kage summoned both the tailed beasts and Team 7 back to the real world, the tailed beasts began to discuss their plans for the future since they were finally free, with Son expressing its desire to return to its cave. Sasuke, however, made his own intentions known: removing as many people as possible who could get in the way of his plans of leading the world through a revolution to do away with current shinobi systems, including killing all the current Kage and tailed beasts. Just as Sasuke swiftly subdued the tailed beasts with his Rinnegan and sealed them within his Chibaku Tensei, Naruto reassured the tailed beasts that he would save them once again. After Naruto was able to defeat and reform Sasuke, the two dispelled the Infinite Tsukuyomi and Sasuke released Son, along with the other tailed beasts, from their prisons.

Video Games
Although Son Gokū itself is not playable, Rōshi takes on its form as his awakening in Naruto Shippūden: Ultimate Ninja Storm 3, Naruto Shippūden: Ultimate Ninja Storm Revolution, and Naruto Shippūden: Ultimate Ninja Storm 4.

Trivia

 * is the Japanese name rendering for 'Sun Wukong', the iconic legendary character from the classic Chinese novel Journey to the West known as the Monkey King and is also used for the character with the same name from the Dragon Ball series. Kishimoto was mainly inspired to create Son based on the aforementioned version of Sun Wukong/Son Gokū, which is referenced in its design resembling the Ōzaru form, the inherited Dragon Ball that has four stars in it, and having a master named Rōshi . Besides the name, Son is also based on several other references to the original version of Sun Wukong/Son Gokū, which includes:
 * The horns on its forehead are modelled after Wukong's diadem.
 * and are both titles that Wukong bears, though the former is written with a different character for "monkey" than from the Chinese version of the title.
 * The is the cave where Wukong was born on the.
 * When Naruto met Son within Rōshi, it was sealed to a giant stone, which reflects the mountain under which the original 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)