Wayfinder

The Lucky Charm is made by Kairi before her departure from the Destiny Islands. The charm is made of seashells (precisely, thalassa shells) that provide good luck for travelers. In Kingdom Hearts: Final Mix, Sora gets a glimpse of the lucky charm while he is still on the Destiny Islands, while he, Riku and Kairi are gathering provisions for the raft.

After Sora rescued Kairi from Hollow Bastion, she gives it to him for luck and makes him promise to return it to her once he gets things settled, which he promises to do.

In Kingdom Hearts: Chain of Memories, the charm is changed into a lucky charm of Paopu Fruit under the influence of Namine's ability to alter Sora's memories. However, it changes back to its original form once Sora remembers Kairi.

If you look at Oathkeeper in Kingdom Hearts II, Kairi's lucky charm on one side and Naminé’s Paopu fruit on the other side. It most probably shows that Kairi and Namine might be different, but they are the same.

During the ending credits of Kingdom Hearts II, Sora returns home and finally returns the lucky charm to Kairi. The lucky charm is also the Keychain for Oathkeeper.