Son Gokū

Son Gokū, more commonly known as the Four-Tails), is one of the nine tailed beasts. It was last sealed within Rōshi of Iwagakure. 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. After Hagoromo's death, the tailed beasts drifted apart, apparently due to Kurama's belief that their number of tails was an indication of their strength. Son Gokū, after living in the Water Screen Cave, 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.

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 "ore" 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. 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. 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.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. 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.

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. 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.