Monkey King: Enma

was the personal summon of Hiruzen Sarutobi, the Third Hokage, and king of the monkeys. As he was a long-time companion to Hiruzen, it is implied that he is also rather old.

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.

Abilities
Enma is capable of using Transformation: Adamantine Staff, which allows him to turn into a large, extendable staff that is as hard as adamantine yet as sharp as a sword. 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 Techniques.

Outside of this, he himself is physically very strong, easily handling the First and Second Hokage 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 First and Second Hokage. With Enma on hand, Hiruzen was able to hold his own until he realized 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 summons that wears a headband, suggesting that his loyalty is strictly to Konoha. The others are the Ninkame of Might Guy and the eight Ninken of Kakashi Hatake.
 * When Enma vanishes after Hiruzen dies he takes the Kusanagi Sword: Long Sword of the Heavens with him. It's unknown how Orochimaru retrieves it.