soraSora is the leading protagonist of Kingdom Hearts. Sora was born and raised on Destiny Islands, and is best friends with both Kairi and Riku - Kairi first met Sora when she was five years old, after Radiant Garden was destroyed. Largely, Sora has always been the upbeat one, the heart and soul of the trio, and often the brunt of many teases due to his good nature; yet, he always shakes them off and takes them in his stride. As a person, Sora has an uncanny ability of making friends wherever he goes, due to his likeable personality. If someone asks him for help, he will give it - sometimes, this is as much a bad thing as a good one, as he will do things unquestioningly, if it's from a source he trusts, or if he feels in his heart that he should do it.
Initially, Sora was not the one destined to inherit the Kingdom Key, the lead Keyblade for the Light - that was to be Riku. However, as Destiny Islands was destroyed, Sora inherited what would have been Riku's destiny as his own, as he did not succumb to the darkness. Sora begins his journey and soon becomes firm companions with Donald and Goofy, whom were looking for the King.
Sora has been targeted by Xehanort on multiple occasions, but as Sora possesses darkness, he attempted to make him turn. Thanks to his friends, Sora is saved and restored, and even though he is not named a Master, is nothing but happy for Riku.
Thinking of you wherever you are.
The first conversation we have in the game is between Sora and Kairi. It soon becomes steadily apparent that there is an underlying current to their friendship - that Sora has an unspoken crush on her, maybe even one he hasn't fully realised. Riku gives Sora a paopu fruit, saying to him that he "knows he wants to try it", and Sora throws it away, flustered. Sora is again flustered when Riku suggests that if he wins a race, that he'll share a paopu fruit with Kairi. It's not the only time, either - on separate occasions in Kingdom Hearts II in Port Royal and Halloween Town, Donald and Goofy tease him about Kairi, and Sora acts in a similar way. Anyway, this conversation with Riku about the paopu fruit prompts Sora into going to The Secret Place. Sora then adds to a picture that he and Kairi drew, by drawing himself giving Kairi a paopu fruit, and if nothing shows that he cares deeply about her, that does. This crush doesn't seem to be just one sided, though - Kairi herself jokingly suggests that they should take the raft and go, just the two of them. And when it comes to Kairi's heart disappearing, and finding someone to keep her safe? She hides within Sora's heart. Let's also not forget how she returns the gesture of the paopu fruit at the end of the first game, touched, and that she lends Sora her wayfinder, her paopu charm, to keep him safe. Sora, likewise, when he sees The Secret Place at the ending of Kingdom Hearts II, is surprised, and then happy, to see that it has been returned to him. There's a lot of unspoken affection and trust between the two of them.
Sora's desire to keep Kairi safe is one of his most consistent and constant endeavours. Her safety is paramount in his mind even if that means she has to stay behind and they cannot be together. It is meant as a kindness, because Sora is very straightforward (Kairi at home equals Kairi being safe), but can just as much be a curse. He spends much of the first game seeking her, and when he does, asks her to stay behind in Traverse Town, and again, to wait on the islands, where he'll come back for her. He is so certain she'll be safe that he is somewhat blindsided when he finds out that Kairi has in fact been kidnapped by Axel, and then, imprisoned by Saix. He even goes to his knees and begs for her to be released; when he is refused, he gives in to despair, if only for a moment, until his resolve to rescue her begins to shine through. That's the thing; Kairi was safe, even if he couldn't find Riku, and her not being safe was just another reminder of a broken promise to find him and to protect her. Even when they reunite, Sora still feels guilt over both of these things - but to Kairi, this doesn't matter, just that he is there and she is able to see him.
Likewise, Sora is ready to give up his own life to keep Kairi safe and to restore her, as he does when he finds out the only way to return her heart is to unlock his own. However, this protection isn't a one way street; Kairi just as much spends her time making sure Sora is safe. She saves him when he turns into a Heartless, fights her way to his side in The World That Never Was, and despairs herself when Sora is put in danger due to her imprisonment. Whatever she can do, she is willing to do it if it helps him in some way.
I really cannot emphasise their importance to each other enough. Be it just protecting one another, unspoken affection, or simply being so important to the other. Even when Sora forgets Kairi due to Naminé's unchaining of his memories, Sora chooses Kairi. He chooses someone he cannot remember. Kairi, likewise, struggles to remember Sora but she does not give in. At the end of the day, it is the faith and strength in knowing the other that ultimately sees them through. They may not have shared a paopu fruit, but their destinies are one, regardless.
In the future, it's unclear if they will become a canon coupling, though I'd say the odds are relatively high. It's hinted, it's there, and there is potential to build on it. As for their relationship, they may not always be together, but they will still think on each other and draw strength from that. As for Sora's desire to keep Kairi safe, her training may not sit well with him for some time; but deep down he'll know she's too involved, that staying at home is no longer strictly safe for her. Being able to protect herself when he cannot will now be the priority.
It's also doesn't hurt to imagine Sora's face when he sees her fighting properly for the first time and she demonstrates her skills with poise and grace.