Jiminy first makes his appearance in Disney Castle and becomes Sora, Donald, and Goofy's royal chronicler under the order of Queen Minnie. He explains to Goofy that he escaped right before his world was destroyed, and he was scattered from his friends. After they befriend Sora, they travel to many different worlds in search for the King, Riku, and Kairi. Some time after, he meets up with Pinocchio in Traverse Town, and tells him to stay put while he sets out to find Geppetto, his father.
When they head for Atlantica/Halloween Town, depending on which world the player chooses, they get swallowed by the giant whale Monstro and meet up with Pinocchio again, this time reunited with his father Geppetto. But then Pinocchio was lured away from his friends by Riku and they searched for him until a Parasite Cage Heartless captured Pinocchio and attempted to take his heart. He failed, but then Riku captured him and stole his heart in hopes to be able to replace Kairi's heart. Jiminy, Sora, Donald, and Goofy manage to restore Pinocchio's heart with the united power of their hearts. Then, they battle Parasite Cage again and defeat it once and for all, but then Monstro feels ill and spit them out and separates Jiminy away from Pinocchio. He finds out that Pinocchio and Geppetto are okay after they visit Traverse Town again and find them living there.
Upon entering Castle Oblivion, Sora and company realize that their memories are disappearing the more they head for each level of Castle Oblivion, and Jiminy's entries in his journal have disappeared altogether.
When Sora gets angry at Donald and Goofy for not wanting to help save Naminé, he sets out to find her by himself. Jiminy tries to reason with him, but he refuses to listen. When they finally meet Naminé, they learn that Sora never knew Naminé, but were rather fooled by her abilities to control memories of Sora and those attached to his heart.
After Sora and Co. defeated Larxene, Vexen, and Marluxia, the members of Organization XIII, Naminé takes them to a room of Memory Pods, where she tells them she will be able to restore their memories while they sleep in them. However, as a price they will lose their memories of all things that happened while they were in Castle Oblivion. Afraid that they might not be able to thank her for what she does for them, Jiminy writes down a small entry in his journal that says "Thank Naminé".
Sora, Donald, Goofy, and Jiminy awake from their year-long slumber in Twilight Town, and have no memories of what happened right after they closed the door to Kingdom Hearts. Goofy asks Jiminy what his journal says, and Jiminy only finds a sentence: "Thank Naminé". Unable to know who she is anymore, Donald scoffs at him for his bad journal entry and Jiminy dismisses it by telling him to go and find out where they are.
After returning to Disney Castle, Jiminy finds a mysterious sentence saying "We must return to free them from their torment" while reorganizing his journal. Jiminy cannot recall any memory of that entry, so he shows the text to King Mickey. To investigate, King Mickey creates a Sora out of data to try and find out more about the mysterious entries that continually appear. As Data-Sora continues to unravel the Bug Blox corruption that is affecting the Datascape, Jiminy somehow gets sucked into the data world, along with King Mickey, Donald, Goofy and Pete. He and his friends are sent back to the real world after the debugging of the journal is complete.
Upon arriving in Prankster's Paradise, Sora meets Jiminy. Despite the efforts of Sora, but he did not remember him at all. Jiminy tells Sora that Pinocchio had told him when he was locked in a cage, that man in a black coat was to play a trick on Sora. Sora then saw off Pinocchio and Jiminy asked to climb on his shoulder. The two friends were the victim of a Jestabocky, who had disguised the appearance of the puppet. They still found themselves in the path Pinocchio dolls, threatened by Dream Eaters. Sora therefore asked to leave Pinocchio with Jiminy. Pinocchio, who learned that his father had been swallowed by Monstro would find at any cost the whale, and Jiminy, fulfilling his role of consciousness, accompanied him. The two friends became so engulfed by the whale.
When Riku met Geppetto in the mouth of Monstro, he promised the old man to find Pinocchio. Jiminy then appeared and decided to accompany the boy. He regained Pinocchio, accompanied by a man in a black coat. It was a double Riku, who personified the part of this young man in the darkness. Riku was troubled by this encounter, but the puppet reassured him by saying that he needed a friend to confide in, as had his Jiminy, which served as the conscience.
Later, Jiminy receives a medal from the Blue Fairy.
True to his name, Jiminy is an anthropomorphized cricket, though he bears virtually no resemblance to real-life crickets. He has olive skin, thin black eyebrows, four-fingered hands, and wears the famous white gloves many older Disney characters wear. He dresses very formally, wearing a royal blue top hat with an orange band near the base, a red vest with a single, pink button on the front, a yellow ascot, a black blazer, and a high-collared white shirt. He also sports tan pants and yellow shoes with black tips. Jiminy also carries a red umbrella.
In Pinocchio, Jiminy serves as the main character's conscience. The Blue Fairy, who brought Pinocchio's puppet body to life, gave Jiminy the task of guiding him and helping him to determine right from wrong. However, Pinocchio's naïveté, more than once, gets the better of him, leading to Jiminy losing his temper several times and scolding him. At one point, Jiminy prepares to abandon Pinocchio altogether when Pinocchio inadvertently chooses a naughty boy named Lampwick over Jiminy, but rushes to the puppet's aid upon realizing he is in danger. Eventually, their friendship prevails and Pinocchio, despite sacrificing his life to save his father from Monstro, is turned into a real boy. Jiminy, for his part, is awarded a golden badge as a sign of his amazing conscience capabilities.