Gerotora

is a scroll toad, a rare species that closely guards whatever secret has been written on its scroll-abdomen until his 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.

Itachi Pursuit Arc
Gerotora first appeared when Jiraiya summoned him, so they could discuss about the key. Jiraiya wanted the scroll to go to Naruto since he believed that he was going to die. 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 its chakra when he used the key to weaken Naruto's seal, which nearly killed Jiraiya. Jiraiya argued that Minato gave him the key's jutsu design as a sign that Minato had intended for Naruto to complete an unknown jutsu. Through this, Gerotora reluctantly agreed. Gerotora was one of the toads (Fukasaku, Gamabunta, and Gamakichi) to visit Konoha to bring news of Jiraiya's death.

Confining the Jinchūriki Arc
Gerotora was 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. Fukasaku asked him to elaborate. Gerotora explained the two parts of the Nine-Tails' power, how the seal and key works, and the risk of having the Nine-Tails being released if Naruto's will 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

 * Beginning with chapter 489, he is called.