Gamabunta

is the boss of the toads that are summoned. He is huge (about 100 meters high) and is summoned for battles with the biggest of enemies. At first, Naruto could only summon him with the Nine-Tailed Demon Fox's chakra. Without it, Naruto could only call upon some of Gamabunta's smallest children. Gamabunta is first seen in the premiere of the series, where he and the Fourth Hokage face off against the demon fox. He is not given a speaking role until he is first summoned by Naruto.

Personality
Gamabunta is a grumpy and highly apathetic character. He doesn't like to take orders from anyone, unless the summoner is highly talented and earns his respect. Jiraiya, Naruto, and the Fourth Hokage are three of the few people that he has allowed to ride on top of his head without question (Naruto had to struggle for Gamabunta to accept him). He uses some Japanese words that only a yakuza gangster would use, but he is portrayed as an ultimately powerful and positive force. He is fiercely protective of his kin and he hates to be underestimated by anyone, both of which take precedence over his normal apathy. Often, it is one or a combination of these two factors that entices him to fight. Overall, despite his grumpy exterior, he shows respect towards Naruto as if he were a son. He even explained to Jiraiya that he's aware of Naruto's talents and potential. He and Jiraiya seem to be good friends and, as implied in the series, drinking buddies, despite Jiraiya's admitted lack of control over Gamabunta. However, he will scold him on occasion.

Chunin Exam arc
When Naruto first summoned Gamabunta, while falling to his death, Gamabunta didn't believe that a little kid like Naruto could summon him. Naruto, angry at the comment, claimed that, as Gamabunta's summoner, he was the toad's master. This made Gamabunta lose his patience with Naruto, and he threatened to kill him. This scared Naruto into submission, but vowed to become the toad's master. It is not until Gamabunta sees the summoning contract scroll that he believes that Naruto actually summoned him, though he is not overly surprised by the fact, indicating that he knew that Naruto was indeed the one who summoned him all along, otherwise the boy wouldn't have persisted to gain his respect. After Naruto passes out from riding Gamabunta trying to make the toad his, the toad boss takes Naruto to the hospital, leaving his footprint as a signature.

Invasion of Konoha arc
During the attack on Konoha, Naruto summoned Gamabunta to save himself from being crushed by Gaara's Sand Waterfall Funeral. Gaara was in his full One-Tailed Shukaku form, though Gaara's personality was still dominant at the time. Gamabunta was his usual grumpy self and refused to fight for Naruto. Gamakichi, who had been accidentally summoned by Naruto earlier, told his father that Gaara tried to kill him and that Naruto had saved him. Gamabunta is impressed with Naruto, and he officially accepts Naruto as his subordinate. At first, Gamabunta was able to hold his own, but this quickly changed when Gaara forced sleep upon himself, allowing the Shukaku to take over. This unleashes the Shukaku's full power, which is too much for Gamabunta to handle. However, Naruto and Gamabunta use a Combination Transformation to take on the form of a giant fox (in the anime the fox has nine tails), allowing Gamabunta to get Naruto close enough to Gaara to wake him up by punching him. Gamabunta was once again impressed with Naruto and even lamented that he would not be able to see the end of this battle because he had reached his limit and had to return home.

Search for Tsunade arc
During the battle between Jiraiya, Tsunade, and Orochimaru, Gamabunta is summoned along with Katsuyu to counter Manda, Orochimaru's giant snake. He seems to have respect for Katsuyu, as he rescued her from Manda, but there is no love lost between Gamabunta and Manda; after being summoned for the battle, Manda threatens to turn Gamabunta into "toad jerky," and Gamabunta responds to Manda's threat by saying that he has always wanted a "snakeskin wallet."

Land of the Sea arc
In the Land of the Sea, Naruto summoned Gamabunta to battle against Umibozu, the Sea Boss. Gamabunta wasn't happy about being summoned in seawater (toads are freshwater creatures), but fought anyway because he was underestimated by his opponent. The sea water also gave him a rash. He and Naruto defeated the otherwise indestructible water creature by evaporating it. Gamabunta then threw Naruto into the air as punishment for summoning him in seawater. However he noticed that he was still happy that Naruto took so much care of his children.

Invasion of Pain arc
Gamabunta makes his Part II debut when Naruto is informed of Jiraiya's death. He, along with Gamaken and Gamahiro, are later summoned to assist in Naruto's fight with Pain. They combat the various animals summoned by the Animal path until they are sent flying beyond Konoha's borders by the Deva path. He is later seen by Neji through the use of the Byakugan, seemingly unconscious on the road back to Konoha. He tells Team Guy that Pain has attacked Konoha when they approached him. He is later seen with the Great Toad Sage.

Shippūden the Movie 2: Bonds
Gamabunta makes a small but comedic appearance at the end of the movie. While still asleep, Jiraiya summons him to cushion the fall of Naruto and Amaru that would have otherwise killed them.

Abilities
In all his appearances, Gamabunta has shown to be a powerful fighter, regardless if whether or not his summoner is by his side. Despite his great size, he possesses surprising agility as seen when he leaped out of the crevice that Naruto first summoned him to, reaching several miles above the atmosphere before crashing back again into the forest.

Gamabunta is a Water Release user, able to 'spit' water ninjutsu such as Water Release: Bullet Technique out of his mouth, making a formidable force against Fire Release users. As well as 'spitting' water jutsu, Gamabunta can also spew oil from his mouth for use in a synchronized collaboration jutsu, called the Fire Release: Toad Oil Flame Bullet technique, in cooperation with his summoner. While his summoner prepares a fire jutsu (or another source of ignition), Gamabunta will prepare a mouthful of oil to fire out of his mouth under high pressure. When Gamabunta is ready, he will shoot the oil out at the target while his summoner (Jiraiya or Naruto), ignites the oil. The oil, acting as an accelerant, fuels the fire, creating an conflagration far greater magnitude and force then the original fire ninjutsu used.

Gamabunta's arsenal includes a large battle knife, which he can wield to inflict colossal damage thanks to his immense strength. Gamabunta also exhibits a wise and knowledgeable demeanor, as seen during the fight against Shukaku where he demonstrates his extensive experience by ordering Naruto to forcibly awaken Gaara.

Trivia

 * Gamabunta wears the kanji for "shrimp" or "ebi" on the back of his kimono of sorts. Ebi, when used with another kanji, turns into "gama," toad.