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. He possesses great physical strength and fighting ability, as seen when he fights off both Hashirama Senju, the First Hokage, and Tobirama Senju, the Second Hokage.

Invasion of Konoha arc
Enma's first appearance was his summoning during the battle with Orochimaru to even the odds against the resurrected First and Second Hokages, now Orochimaru's minions due to his Summoning: Impure World Resurrection. 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 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."

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.

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 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 Enma uses for Adamantine Prison Wall.

Outside of this, he himself is physically very strong, easily punching the Second Hokage through several feet of wood and in two moves knocking the First Hokage away, giving himself enough time to transform.

Trivia

 * Of all the summonings to date, Enma is one of the only three to wear a hidden village symbol of any kind (Ninkame, Guy's summon, being the second and Kakashi Hatake's Ninken being the third), thereby implying that their loyalty is strictly to Konoha.
 * Some fans have speculated that Enma is a personal summoning of the Sarutobi clan and will make a reappearance through Konohamaru.