Gerotora

is a scroll toad from Mount Myōboku — a rare species that closely guards whatever secret has been written on its scroll-abdomen until its contractor dies. He resides in the belly of his contractor, and is known for his excellent endurance.

Background
Gerotora guards the scroll which acts as the holder of the "key" to the Eight Trigrams Seal located on Naruto Uzumaki's stomach. He was summoned during the Nine-Tailed Demon Fox's attack on Konoha by Minato Namikaze, who inscribed the key onto his stomach for safekeeping. Minato left the key to Jiraiya upon his death.

Personality
Gerotora is shown to be a highly sceptical individual, refusing to believe someone from the founding days of Konoha could live well beyond their time from the founding of the village to the term of the Fourth Hokage.

Appearance
He is a black and orange toad with dark markings around his eyes, mouth and a circular mark just below his mouth. As a scroll toad, his abdomen can extend to reveal a scroll which when concealed, gives him the appearance of wearing an obi.

Interlude
About two years after Naruto started training under Jiraiya, Gerotora was summoned again. As Jiraiya began helping Naruto gain better control of the Nine-Tails' power, teaching Naruto to control its demon cloak up to producing two chaka tails, Jiraiya decided to have Gerotora unleash a bit more chakra. Gerotora was strongly against this action, but Jiraiya insisted they were ready. Gerotora did as he was asked, only for the plan to backfire. The Nine-Tails took advantage of this moment to force out more of its chakra, forcing Naruto into a "Version 2" state with four tails and seize control of Naruto's body. As Gerotora noted he was right to worry about this idea, Jiraiya quickly went into action to reseal the chakra and save Naruto.

Tale of Jiraiya the Gallant
Gerotora was summoned later by Jiraiya so they could discuss the key to the seal left on his abdomen by Minato. Jiraiya wanted the scroll to go to Naruto since he believed that he could die during his upcoming battle. Gerotora angrily refused, believing that Naruto wasn't ready, since during Naruto's training, Jiraiya failed to teach Naruto to filter out the Nine-Tails' will when he tapped into the beast's chakra having used the key to weaken Naruto's seal a little nearly killing Jiraiya in the aftermath. Jiraiya argued that he was given the key's technique design as a sign that Minato had intended for Naruto to complete an unknown technique. Eventually, Gerotora reluctantly agreed as Jiraiya departed.

Pain's Assault
Gerotora was present when Naruto was informed of Jiraiya's death. He listened on as Fukasaku told everyone about Pain, Jiraiya's killer as well as Naruto's emotional outburst.

Fourth Shinobi World War: Countdown
Gerotora was later summoned by the Great Toad Sage and was told to give Naruto the key to the Eight Trigrams Seal. Gerotora said that he didn't blame Naruto for being hesitant on using the key. When Fukasaku asked him to elaborate, Gerotora explained the details of the seal, as well as the risk of having the Nine-Tails being released if Naruto's willpower wasn't stronger than the Nine-Tails'. Naruto, remembering what had happened when the Nine-Tails took over him, said that he would need the Nine-Tails' chakra in order to fight Sasuke Uchiha, so he then signed the contract. Gerotora proceeded to enter Naruto's belly through his mouth, although it took Fukasaku kicking Gerotora in to get him down Naruto's throat.

Trivia

 * Though his name was given as Gerotora in the third databook, from chapter 489 onward, he was instead referred to as . In the fourth databook, he was once again called Gerotora.
 * "Gero" is a Japanese onomatopoeia for the sound a frog makes.