Son Gokū

, better 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 resurrected 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' life in order to prevent the Ten-Tails' resurgence, the Sage split up its chakra and used his ability known as to create nine separate constructs of living chakra, which would become known as the tailed beasts. 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 was.

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

Personality
From what has been seen of Son Gokū, it detests being called by the name "Four-Tails". It is also a very prideful tailed beast boasting about who it was and its lineage calling itself the "Handsome Monkey King of the Water Screen Cave; the King of the Sage Monkeys, bestowed with the Dharma name of Son by the Sage of the Six Paths. The Great Sage Equalling Heaven".

It also seems to detest humans a great deal believing that apes are more intelligent than humans. This hatred seems to stem from that fact that humans have enslaved it. However, when talking about the Sage of the Six Paths, it showed him 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 going away.

With Rōshi, it didn't seem to get along all that well with him due to knowing him for forty years and Rōshi has never once called it by its given name. In the end, Rōshi wished they could have gone down the same path like Naruto and reconciled with the beast calling it by its name before fading 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, 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 then and had its teeth less large than now.

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. Since it is a tailed beast, it is able to use the Tailed Beast Ball. Son Gokū is also shown to be very strong as it was able to lift and toss Gyūki.

Activity inside Rōshi
As Son Gokū's jinchūriki, Rōshi could borrow its ability to create and use lava.

Itachi Pursuit Arc
After an unseen and difficult battle with Kisame, Rōshi was somehow defeated and captured by him. Kisame had difficulty capturing Rōshi because of his power over lava. Son was later extracted and sealed, leading to Rōshi's death.

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

Now inside the beast, a chained up Son asks Naruto if he had come to steal its power as well, and lashes out at him when he refers to it as the "Four-Tails" telling him that it was the king of the apes and would be addressed with due respect. As their conversation continues, Son finds Naruto to be an open, honest person which shocked him somewhat. After cursing that all humans did was seal them away and deny their existence, Naruto shares 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, it realises that he is serious and tells the boy that there might be a way to stop him if the chains that bound it were undone.

As Naruto, who is eager to help, asks how Son tells him beforehand that he's made it a point not to trust jinchūriki and not to expect for it to become his ally unless he trusted him. As Son is forced to vomit Naruto out after the latter used the Multiple Shadow Clone Technique, the beast tells Naruto of the location of the chakra receiver that was embedded into him which had moved since he had gotten bigger. While still being manipulated to attack Naruto, a Naruto clone who had stayed inside the beast's mouth prepares a two-fold attack on the chakra receiver after going into Sage Mode. As the manoeuvre succeeds and the receiver is removed from Son's body, Son notes that the chain still binds him. As Naruto argues with him over wanting to save him completely and not just free him from Tobi's control, Son in somewhat disbelief offers some of his chakra to Naruto, pleased that he truly tried to save him. Son is 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 is revealed that Rōshi attempted to make amends with the beast, wondering whether or not they could have walked the same path as Naruto would. Son tells if that maybe if he wasn't as stubborn as Ōnoki and forgotten its name all those years ago, it might have been possible. As the beast is resealed into the Demonic Statue, it parts on good terms with Rōshi who asked if calling the beast "Son" would be all-right.

Conception
The design of Son Gokū is a combination of a gorilla and the Ōzaru from Dragon Ball. The horns on its forehead are modelled after the diadem worn by Sun Wukong from the classic Chinese novel Journey to the West, the character that the similarly named Son Gokū who could also transform into an ape and inherited a 'dragon ball' with four stars on it, was based on. And his name Son Gokū is the Japanese name for Sun Wukong.

Trivia

 * Likely owing to the inspiration Son takes from Journey to the West, when Naruto meets him within Rōshi, he is sealed to a giant stone. This reflects the mountain under which Sun Wukong was sealed.
 * and are both titles Sun Wukong from the Journey to the West bears.
 * The is the cave where Sun Wukong was born on the.
 * 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

 * (Son introduces itself)