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. When he, Donald and Goofy enter the End of the World, Sora takes a look at Kairi's lucky charm, remembering his promise to return it to her.

In Kingdom Hearts: Chain of Memories, the charm is changed into a lucky charm of Paopu Fruit under the influence of Naminé's ability to alter Sora's memories. However, it changes back to its original form once Sora remembers Kairi. Towards the end of the game, Sora looks at it to remember Kairi's name, when Naminé tells him that she is "the light in his darkness".

In the prologue of Kingdom Hearts II, Kairi is seen threading the lucky charm and contemplating it on the sands of the Destiny Islands, leaving one lone shell to be washed away by the water. Right before 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. If you look at the Oathkeeper keychain in Kingdom Hearts II, you will see Kairi's lucky charm on one side and Naminé’s Paopu fruit on the other side. Both charms have the shape of a 5-point star. It most probably shows that Kairi and Naminé might be different, but they are the same.