Cid is a character originally from Final Fantasy VII, who makes an appearance in all Kingdom Hearts games except for Kingdom Hearts 358/2 Days and Kingdom Hearts Birth by Sleep. Cid is 42 years old at the beginning of Kingdom Hearts and Kingdom Hearts: Chain of Memories and 43 during the events of Kingdom Hearts II
Cid is one of the first characters that Sora meets and befriends after arriving in Traverse Town. He runs an accessory shop in the center of the town's 1st District. At one point, Cid sends Sora out to deliver an old, broken book—The Adventures of Winnie the Pooh—to Merlin. When Sora and company return to Traverse Town with unusual gummis, he explains Navigation Gummis to them and offers to install that ones they find on their Gummi Ship. He turns over the accessory shop to someone else, and he begins selling Gummi Ship parts outside the synthesis shop. This is true to his Final Fantasy VII counterpart, as he is an expert on air travel. In fact, most incarnations of Cid in the Final Fantasy series have been associated with airships and mechanics.
Like Leon, Merlin, and other important characters met in Traverse Town, Cid was originally from Radiant Garden. When Leon, Yuffie and Aerith depart to Hollow Bastion, he remains behind in Traverse Town, and seemingly becomes Kairi's only guardian. After Sora sealed Kingdom Hearts, he returns to his world, meeting up with Cloud on the way.
Cid appears in Castle Oblivion, in the memory-based version of Traverse Town. Like the other characters appearing out of Sora's memory, he doesn't remember Sora, but knows his name.
Cid is a member of the Hollow Bastion Restoration Committee, working out of Merlin's home. He fills Sora and company in on the goings on in town when the drop by. Cid seems to have a rivalry with Merlin, with Cid favoring technology over magic and Merlin favoring magic. He is a computer expert, and manages to create a program which helps Sora and Tron defeat the Master Control Program.
Shortly after entering the Datascape's version of Traverse Town, Data Sora meets Cid outside the accessory shop. Cid asks Sora to help search for Huey, Dewey, and Louie around town. After Sora brings back Huey and Dewey, Cid gives Sora a piece of the world's Keyhole.