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. His sons are Gamakichi and Gamatatsu. Naruto can only summon him with the Nine-Tailed Demon Fox's chakra. Without it, Naruto can only call upon some of Gamabunta's smallest children. Gamabunta was 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 in the month long training session with Jiraiya. Jiraiya pushed him over a cliff in order to induce life-threatening fear in Naruto so that the demon fox chakra could come out, but never expected Naruto to summon Gamabunta. Given that Gamabunta wasn't happy about appearing at the bottom of a canyon, Jiraiya quickly fled.

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, Even though Gamabunta looks like a "slacker" he is one of the most powerful ninja summons out there.

Chunin Exam
When Naruto first summoned Gamabunta, 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 the giant summon's respect; this could also mean that he was just testing out the boy's feistiness as his new summoner, since it's implied it has been a while that someone other that Jiraiya, as well as the Fourth Hokage, has shown the ability to summon him. 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
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 in the Japanese version told Naruto that they couldn't have a subordinate/mentor relationship until they shared sake, despite the fact that Naruto wasn't old enough to drink. In the English version Gamabunta said that there was no signed contract signifying Naruto as his henchman so he wouldn't help him and Naruto protested that he wasn't old enough to sign anything (despite him having already signed a contract with the other toads). 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 the Nine-Tailed Demon Fox, 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.

Search for Tsunade
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. He doesn't get along anywhere near so well with Manda. Manda threatens to "pop" Gamabunta, and Gamabunta threatens to make Manda into a "snakeskin wallet".

Sea Country
In the Sea Country, 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. 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.

Post Hunt For Uchiha arc
Gamabunta makes his Shippūden debut along with his son Gamakichi when Naruto is informed of Jiraiya's death.

Abilities
In all his appearances, Gamabunta has shown to be a powerful fighter, regardless 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 fell in right before he summoned the great Toad, reaching several miles above the atmosphere before crashing back again into the forests. Gamabunta's arsenal includes two katanas, both of which he could wield to inflict colossal damage thanks to his immense strength, and appears to be fairly proficient in certain forms of Ninjutsu, though he seems to favor Water Release attacks over anything during confrontations. Also, being possibly one of the eldest summons in existence, Gamabunta 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 is the first summon seen in the series, although at the time, he was not classified as a summmoned animal.