Monkey King: Enma

was the personal summon of Hiruzen Sarutobi, the Third Hokage. He was a long-time companion to Hiruzen, implying that he is also rather old.

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

Personality
Enma was fiercely loyal to Hiruzen, although he didn't always approve of his actions. He kept reminding Hiruzen about actions in regards to Orochimaru. Enma would have preferred to have killed him the first time. Enma was seen in a flashback of Orochimaru's defection, urging Hiruzen to kill Orochimaru, which explains his current attitude. He is also quick to anger, as shown when he tried to break Orochimaru's neck, right after Orochimaru injured Hiruzen.

Appearance
Customary of most monkeys, Enma's body and tail is covered by white fur which protrude from his sleeves and pants. He has long white hair that reached into 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 sash. Enma also wears a Konoha forehead protector.

Abilities
Enma is capable of using Transformation: Adamantine Staff, which allows him to turn into a large, extendible staff that is as hard as adamantine, though despite its hardness, Enma has said that Orochimaru's Kusanagi sword can cut him. He can even push Kurama out of the village rather easily. He can also 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.

Outside of this, he himself is physically very strong, easily handling Hashirama and Tobirama Senju in brief hand-to-hand combat when they tried to keep him from transforming. He also shows knowledge of various jutsu and their effects or drawbacks.

Invasion of Konoha Arc
Hiruzen summoned Enma during his battle with Orochimaru, to help combat the revived Hashirama and Tobirama. With Enma on hand, Hiruzen was able to hold his own until he realised that his resurrected opponents would never die. Therefore, Hiruzen decided to seal both their souls and Orochimaru's, ending their threats forever. As Hiruzen attempted to seal Orochimaru's soul, Orochimaru tried to plunge his Kusanagi sword through Hiruzen's chest. At the same time, he restricted Enma with a number of his snakes. As a result, Enma was only able to catch the sword after it had impaled Hiruzen. He held it back until Hiruzen had sealed Orochimaru's arms, at which point he pulled out the sword before vanishing, noting that Hiruzen had died a "true shinobi warrior" and a "great Third Hokage".

Trivia

 * Enma is one of ten summoned animals that wear a forehead protector suggesting that his loyalty is strictly to Konoha. The others are the Ningame of Might Guy and the eight ninken of Kakashi Hatake.
 * When Enma vanishes after Hiruzen dies he takes Orochimaru's Sword of Kusanagi with him. It's unknown how Orochimaru retrieves it.
 * Enma is a caricature of the legendary Chinese Monkey King, Sun Wukong. Sun Wukong carries a staff which grows and shrinks at will and wears the pelt of a legendary tiger as well.
 * In Buddhism, Enma is one of the twelve Deva. He's chief judge in the afterlife, when a person dies they must appear before Enma (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, Enma is the most important.

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."