Akamaru

is Kiba Inuzuka's, best friend, and companion.

Background
At some point while Kiba was still in the academy, his mother, Tsume Inuzuka, entrusted him with Akamaru. Although Akamaru almost immediately urinated on Kiba's face, much to Tsume and Hana Inuzuka's amusement, the two grew close.

Sasuke Retrieval arc
Over time, Kiba and Akamaru develop Dynamic Marking and Double Wolf Fang together. In the mission to retrieve Sasuke, Kiba and Akamaru remain with the group until they retrieve the coffin, but a paper bomb blows them and Sakon and Ukon into a canyon. Kiba and Akamaru initially struggle against the brothers, and Akamaru realizes that they must use Double Wolf Fang. Kiba is hesitant, given the large amount of chakra needed, which would render them vulnerable if they miss. Akamaru bites him on the hand, and Kiba, realizing that Akamaru sees him as pathetic, decides to take the risk. The attack severely wounds Sakon and Ukon, but they block the last attack with a Rashomon barrier, and Akamaru is injured protecting Kiba from their counterattack, but urinates in Sakon's eyes. When Ukon starts taking over Kiba's body, he stabs himself in the chest in an attempt to kill Sakon to prevent himself from being taken over and harming Akamaru. Kiba attempts to escape with Akamaru, and while Sakon and Ukon catch up with them, Kankuro arrives and kills both of them.

Kiba and Akamaru's injuries take some time to heal. Kiba, not wanting Akamaru to suffer that much again, vows to improve himself.

Filler arcs
Akamaru accompanies Kiba on his missions with Naruto in the fillers. In the Treasure Hunt arc, his presence is used as Kiba's proof of being the real Kiba, as while his impostor could imitate his face, he could not replace Akamaru.

In Episode 184, Akamaru is infected with a toxin that causes him to go berserk. When he attacks some ANBU members who are training, he is ordered to be taken into custody. As Hana attempts to make a cure for Akamaru, Kiba smuggles his dog out of the village and escapes, having heard that Akamaru might need to be killed, but Akamaru continues to lose control of himself. By the time Naruto and Hana catch up with Kiba, Akamaru has turned into a giant version of himself, and attacks anyone near him. Kiba hesitates in the face of the possibility that Akamaru might die, but after Naruto and Hana's attempt to pin Akamaru down and administer the antidote fails, Kiba apologizes to Akamaru while standing still as Akamaru jumps at him. Kiba is severely injured, but manages to inject Akamaru with the antidote, and notes that Akamaru tried his best to avoid killing him.

Sai and Sasuke arc
When Naruto meets Kiba while looking for teammates, he notices that Akamaru is much larger, especially given that Kiba now rides on him rather than having him sit on his head. Naruto asks Kiba to join, and even proposes having Akamaru go with him without Kiba, but Kiba refuses the suggestion.

Fire Temple filler arc
When Team Yamato returns from the Fire Temple with a young monk named Sora, Sora quickly makes it known he doesn't care what any living thing thinks of him. He quickly gets into a staring contest against Akamaru, to which Kiba gets annoyed at. When Furido and his men attack the Leaf Village, Akamaru demonstrates that his new size also helps considerably in battle as he is able to take down several zombies without transforming.

Three-Tails Arrival filler arc
When Kakashi is ordered to investigate the lastest wherabouts of Orochimaru, Team Kurenai is assigned to work with him do to their tracking skills. When they manage to find Orochimaru's hideout, they trigger a trap that destroys the base. Fortunately, Akamaru's nose was able to find a way out in time. Later, Akamaru begins to have trouble concentrated due to a high-frequency sound. It is through this that the team realizes that they being spied on by bats. When the enemy ambushed them with smokescreen, Akamaru convinces Kiba to try their beast-like taijutsu to blow away the smoke.

Soon, the team finds themselves attacked by Guren. There, Kiba and Akamaru again work together to fight of her attacks.

Hunt for the Uchiha arc
After it is decided that the best way to find Sasuke is to find Itachi, Akamaru and Kiba later assist Naruto in searching for either of the Uchiha brothers, but they are unable to find either and return home.

Pain's Invasion arc
When Pain invades Konoha, Kiba and Akamaru join Kiba's mother in fighting the Preta path.

Abilities
In Part I, Kiba carries Akamaru around on top of his head or inside his jacket. In Part II, Akamaru grows large enough for Kiba to ride on his back, a difference Kiba fails to notice due to the amount of time they spend together. Akamaru's heightened senses, such as smell and hearing, along with Kiba's ability to communicate with him, make him a powerful ally for gathering and sharing information; he can also sense chakra with his nose, so he can judge an enemy's strength and warn Kiba if they are particularly powerful. In combat, Akamaru typically takes Kiba's form (albeit having a much more feral appearance) upon Kiba's use of Beast Human Clone, or combining with Kiba into a single entity in more desperate situations. Because Akamaru lacks his own chakra reserves Kiba performs many of the jutsu that Akamaru takes part in, though Akamaru is able to contribute when it comes to physical attacks. Akamaru can also use Dynamic Marking by peeing on a target with great accuracy, allowing either Kiba or Akamaru to easily find the target by scent, particularly when fused together.

Trivia

 * Aka means red. -maru is a suffix for Japanese male names. The reason for his name is explained by Kiba when Akamaru's fur turns red after eating a soldier pill.
 * Akamaru's breed is similar to a Great Pyrenees.
 * His favorite food is the special dog food Kiba makes.
 * Akamaru is not to be confused with Kakashi Hatake's nin-dogs. Akamaru is an Inuzuka dog and is not a summoning.
 * In an omake segment that aired in Japan for New Year's, Kiba attempts to get Akamaru to talk, with little success. There are no indications that Akamaru even has the potential to speak.
 * Interestingly, Naruto was first published in a segment of Weekly Shonen Jump known as "Akamaru Jump."