Monkey King: Enma

was the personal summon of Hiruzen Sarutobi, the Third Hokage.

Background
In the anime, during Hiruzen's time as the teacher of the Sannin, Enma was summoned during their training sessions.

At some point in time when Hiruzen went to accost Orochimaru who had fallen under suspicion, Enma was summoned when Hiruzen decided to confront him. During the confrontation however, Hiruzen allowed Orochimaru to escape despite Enma's appeal to kill him.

During the Nine-Tailed Demon Fox's attack on Konoha, he fought alongside Hiruzen against the beast. He and Hiruzen also later witnessed the Nine-Tails being sealed inside of Naruto Uzumaki.

Personality
He is a long-time companion of Hiruzen, implying that he himself, is also rather old. Enma was fiercely loyal to Hiruzen, although he didn't always approve of his actions. He kept reminding Hiruzen about actions with regards to Orochimaru. Enma would have preferred to have killed him the first time when he got the chance to do so. Enma was seen in a flashback of Orochimaru's defection, urging Hiruzen to kill Orochimaru, which explains his current attitude. He is also very defensive of Hiruzen, as shown when he tried to break Orochimaru's neck, right after Orochimaru injured the Hokage.

He also seems to be quite well-versed of Konohagakure's affairs, as immediately after being summoned during the Nine-Tails' attack on the village, Enma immediately realised that Kushina Uzumaki's seal containing the Nine-Tails had been broken.

Appearance
Enma's body and tail is covered by white fur which protrude from his sleeves and pants. He has long unkempt white hair that reached his back and long sideburns and a goatee. He wears a black suit with mesh armour underneath, over which he wears a sleeveless kimono-shirt with white fur trimmings, and markings reminiscent of tiger stripes on it, which is held closed by a red sash. Enma also wears a Konoha forehead protector.

Abilities
Enma is a powerful summon who is also highly experienced and knowledgeable of various techniques and their effects or drawbacks. Physically he is also very strong, and was easily able to incapacitate a reincarnated Hashirama and Tobirama Senju in brief hand-to-hand combat when they tried to keep him from transforming under the command of Orochimaru.

Ninjutsu
Enma is capable of using Transformation: Adamantine Staff, which allows him to turn into an extending and expanding staff that is as hard as adamantine, though despite its hardness, Enma has said that Orochimaru's Kusanagi sword can hurt him. By extending in staff form, Enma can even push Kurama out of Konoha rather easily. He is also able to produce various body parts, such as eyes or arms, from the staff. Additionally, Enma can still move on his own in the staff form, allowing him to return to Hiruzen. Enma can even clone himself in his staff form, turning him into any number of smaller staffs that he uses for Adamantine Prison Wall, a move that can even protect against Hashirama Senju's Wood Release techniques.

Konoha Crush
Hiruzen summoned Enma during his battle with Orochimaru, to help combat the reincarnated Hashirama and Tobirama Senju when the former had used his Wood Release Secret Technique: Nativity of a World of Trees technique in an attempt to crush Hiruzen. Enma quickly formed a lattice around Hiruzen, protecting him from the attack. With a transformed Enma in hand, Hiruzen was able to hold his own until he realised that his reincarnated opponents were indestructible as the battle waged on. Given no choice but to use the Dead Demon Consuming Seal to seal away his predecessors' souls at the cost of his own, Hiruzen attempts to do the same with Orochimaru so he would never harm Konoha again. However, while attempted to seal Orochimaru's soul, Orochimaru tried to plunge his Kusanagi sword through Hiruzen's chest, but a restricted Enma was able to grab onto the sword and prevent it from killing him until Hiruzen had sealed Orochimaru's arms. With the deed done, Enma pulled out the sword before vanishing as a sign of respect, noting that Hiruzen had died as both a true shinobi, and a great Hokage.

Birth of the Ten-Tails' Jinchūriki
When the Ten-Tails transformed into its tree form, a reincarnated Hiruzen summoned Enma who transformed into his staff form to help him free Naruto Uzumaki. Enma, alongside Hiruzen and the Hokage, later watched on as a transformed Naruto and Sasuke launched another assault on Obito. Later, while the tree is stopped by Orochimaru's snakes, Hiruzen wielded Enma in his staff form to destroy the tree's branches.

Trivia

 * When Enma vanishes after Hiruzen dies, he takes Orochimaru's Sword of Kusanagi with him; it is unknown how Orochimaru retrieved it.
 * Akin to Hiruzen, Enma is a caricature of the legendary Chinese Monkey King,, via the golden-rimmed-staff motif (that nods to his iconic staff that can shrink and grow), as well as following his clothing scheme via wearing a similar tiger pelt akin to Wukong's.
 * Enma's name is a pun off of the Japanese name for Yama (though written in different characters); in Buddhism, Yama is one of the twelve Deva. He's chief judge in the afterlife, when a person dies they must appear before Yama (and also before other judges), who decides whether the person is good or bad; the person is then sent to the most appropriate afterworld. Among the judges of hell, Yama is the most important.
 * In both the anime and manga, Enma is summoned during the Fourth Shinobi World War by Hiruzen, which shouldn't be possible, since the only way Hiruzen could summon him was through the Summoning Technique, which requires Hiruzen's blood. At the time, Hiruzen was a reanimation and therefore unable to bleed, leaving the manner of how Enma came to be summoned unknown.

Quotes

 * (To Orochimaru) "It's over. I'm going to snap you like a twig! It should have been done years ago!"
 * (Talking about the Third Hokage) "To follow the path of the shinobi to the very end… Was something that only you would do."