Monkey King: Enma

is the personal summon of Hiruzen Sarutobi, the Third Hokage, and king of the monkeys. As he is a longtime companion to Hiruzen, it is implied that he too is rather old.

Personality
Enma is fiercely loyal to Hiruzen, though he doesn't always approve of his actions. Hiruzen's actions in regards to Orochimaru is something Enma keeps reminding Hiruzen of. Enma would have preferred to kill him the first time. Enma is seen in a flashback of Orochimaru's defection, urging Hiruzen to kill Orochimaru, which explains his current attitude. He's also quick to anger, as seen when he simply tries to break Orochimaru's neck after Orochimaru injures Hiruzen.

Invasion of Konoha arc
Hiruzen summons 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 sealed Orochimaru's arms, at which point he pulls out the sword before vanishing, noting that Hiruzen died a "true shinobi warrior" and a "great Third Hokage."

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 Hokages in brief hand to hand when they tried to keep him from transforming. He also shows knowledge of various jutsu and their effects or drawbacks.

Trivia

 * Enma is one of three summons to wear a headband, suggesting that his loyalty is strictly to Konoha. The other two are Ninkame and the Ninken.