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 "key" to the Eight Trigrams Seal located on Naruto Uzumaki's stomach. Minato Namikaze left the key to Jiraiya upon his death.

Hunt for Itachi 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, and pointed out that the key was essentially being left upon the very "safe" it opens. 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 is summoned by the Great Toad Sage and is told to give Naruto the key to opening and closing the Fourth Hokage's seal. He tells Naruto that with the key he will be able to release the Nine-Tailed Demon Fox completely if he wanted to, by putting his hands on the square. He goes on telling Naruto that the "key" is also the ticket to perfecting the aforementioned jutsu that Jiraiya had once spoke to him of.

Gerotora told Naruto that he didn't blame him for being nervous. Fukasaku asked him to elaborate. Gerotora explained that there are two parts of the Nine-Tails' power: the chakra and the will. To command its power, one needed to pull out its chakra and make it your own. However, Gerotora explained that when one tries to take its chakra, the will of the Nine-Tails will come along with it, a malevolent hate. The only way to control the Nine-Tails is to have a strong enough will with no hatred, and filter out the hate from the chakra. He then explained that the Fourth Hokage's seal allowed a tiny part of the chakra to naturally mix with Naruto's, and that with the key to the seal, Naruto would be able to tap into all of its chakra, along with all of its mind. Fukasaku asked about the time Gerotora tried opening the seal. Gerotora explains that the seal got weaker over time, and that Naruto would start turning into the Nine-Tails out of nowhere, so Jiraiya would usually use the key to suppress it. He proceeded, explaining that Jiraiya tried to open the seal just a bit, to try teaching Naruto to filter out its mind when he tapped into its chakra. Naruto went on to explain what happened when the Nine-Tails took over, and then signed the contract, prompting Gerotora to say he was Naruto's now. 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.