Paopu Fruit

A Paopu Fruit is a star shaped fruit native to Destiny Islands. It resembles the real life star fruit.

Kingdom Hearts
In an early scene in Kingdom Hearts, Riku explains the fruit to Sora while on the bridge in Destiny Islands. Riku tosses him one and tells him a legend that says that if two people share the same Paopu Fruit, they will remain a part of each other's lives forever. Later on, while racing for the right to name the raft they have built, Riku raises the stakes. He says that the winner should get to share a Paopu fruit with Kairi (though if asked about it after the race, he will state he was merely joking). This intensifies the rivalry between the two, both to prove who is stronger and to gain Kairi's attention. Selphie thinks the legend is very romantic and wants to try it, sometime. Later, in the Secret Cave, Sora draws a picture of himself giving Kairi the fruit. This same drawing touches Kairi in the end credits of Kingdom Hearts (she is seen shedding a few tears) and draws another picture of herself giving the fruit to Sora.

Kingdom Hearts: Chain of Memories
In Kingdom Hearts: Chain of Memories, the fake good luck charm Naminé gave to Sora in Castle Oblivion, also resembles a Paopu fruit. This good luck charm returns to its true form when Sora begins to remember Kairi.

Kingdom Hearts II
In Kingdom Hearts II, the Paopu Fruit is the keychain on Kairi's Keyblade. During the end credits, Sora discovers that Kairi had added onto the picture that he drew in the first game; the picture now depicts Sora and Kairi exchanging a Paopu fruit.

Trivia

 * When looking at the Oathkeeper, when Sora wields it, the keychain is Kairi's Lucky Charm (aka: the Thalassa Shell charm). However, in Roxas' possession, the keychain is Namine's Paopu Fruit from Kingdom Hearts: Chain of Memories. This could possibly be this way because both keychains represent the closest person to the Keyblade wielder (The seashell represents Sora's feelings for Kairi as the Papou fruit represents Naminé's feeling for Roxas).