The legend of the paopu fruit is first introduced in the opening few minutes of the game. Selphie mentions it to Sora, and later, Riku brings it up as his self claimed prize if he wins the race against Sora. The legend can be summarised as, if you share one with someone else, you'll be a part of that person's life forever. Sora takes this message to heart, and shortly afterwards, he goes to The Secret Place, to look at a drawing he drew as a child, of him and Kairi - he then draws himself giving a paopu fruit to Kairi. Kairi, when the islands are reformed, reciprocates this gesture. Whilst they may not have shared one in actuality, figuratively, they have. It's a sign of their devotion and trust for the other, and it's very symbolic, especially for the islanders. As well as the actual fruit, Kairi takes up the mantle of making a charm from thalassa seashells she finds on the beach. The charm is itself shaped as a paopu fruit - a star - and she terms it as her "lucky charm". Kairi gives it to Sora on many separate occasions, and it gives him strength by its mere presence alone - especially when, as Kairi states in the first game, they made it this far by sticking together. Even if she's no longer within his heart, part of her is. Her charm stays with him, and he is often seen looking at it, thinking of her.

Chronologically, Kairi is not the first to come up with the idea of a lucky charm in the shape of a paopu fruit. Perhaps the best name for this charm is a wayfinder, courtesy of Aqua. Whilst in The Land of Departure, Aqua made wayfinders - blue, green, and orange - for Terra and Ven as well as for herself. Whilst they would grow and change and probably go on separate missions, if they had their wayfinders they would always be connected, no matter what; and, as the name suggests, should they lose their way, she would hope that it would help them find it again. Like Sora ten years later, this trio are often seen looking at their respective wayfinders and thinking fondly of the other two and furthering their resolve by doing so. For Sora it is much the same - it guides him, keeps him strong, a token of his relationship with Kairi (though perhaps a little more intimate for the pair of them). Anyway, whilst Aqua comes up with the wayfinder, she mentions that she hasn't seen the star fruit tree, just heard the legend of it. Similarly, Kairi is unaware that she is making a wayfinder other than that it'll keep them safe and she is mimicking a paopu fruit intentionally. It's unlikely she knew of Aqua's own wayfinders, since she never saw any of them. It's a coincidence, yes, also given that she inherits Aqua's Keyblade - but one that perhaps shows how strong their hearts are.

Kairi's wayfinder has been passed between her and Sora multiple times presently, in universe, Sora still has it since he took it with him for the Mark of Mastery exam, and when Kairi arrived at the Mysterious Tower, Sora had returned to the dream world to thank his Dream Eaters. It's probable that it may pass again between them a couple more times as Kingdom Hearts III begins; and it's also possible that, given that Birth by Sleep was released long after the first game, Kairi may decide - on a whim - to make a charm for Riku as well as herself. Not the same as Sora's, probably, as that's special; but enough to keep all of them strong in the search for the other Guardians.