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 body 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. The key was left to Jiraiya by Minato Namikaze upon his death.

Hunt for Itachi arc
He first appeared when Jiraiya summoned him, so they could talk about Naruto's 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" would essentially be 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 as-of-yet 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.

Fourth Shinobi World War 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.

Trivia

 * In chapter 489, he is called.