Harsh Reality of Destiny Trio
The more I look at Destiny Trio, the more I believe they will not have a good ending at the end of KH4. The harsh truth is that Destiny Trio are never true friends. They're only bonded by Sora. Riku and Kairi, as well as their relationship, are clear indications of a fragile bonding waiting to be broken down.
Ever since KH1, there was a visible friction between all of them. Kairi wanted to run away with Sora, and Riku was all alone with his dark thoughts. As KH2 and KH3 went on, three of them tried to be together and failed. Sora and Riku became more proactive and closer, while Kairi became more distant. Kairi tried to grab whatever made her bonding with Sora meaningful. She tried to fight for Sora and paid a high price for it. When Sora rescued her and lost his life, Kairi felt a huge guilt and became more desperate. That was until she got sidelined and scolded by Riku who thought she was worthless and hindrance. This is a direct flashback to Sora's "you'll only get in a way", bit only harsher. Because Kairi knows she wasn't strong enough and Riku now hated her for blatantly costing Sora's life.
The takeaway from this is that Destiny Trio became less friends and more of symbiotes. Their friendship dissipated long ago. Only Sora acted as a catalyst to bond their fragile relationship and mask it as a friendship. Without Sora, Riku and Kairi would've never became friends. And with Sora gone, there's no excuse for either of them to act friendly anymore. There's clearly a tension brewing up behind the scenes. Riku is exhausted and scared of rescuing a friend who threw away his life for one girl. Kairi is haunted and depressed by being powerless and hated by everyone while she continues to do meaningless training and self-reflection.
As Destiny Trio grows desperate to restore their old ways, they end up being more segregated and separated by their own ideology and emotion. Their destiny is to be separated for eternity. In one way or another, they can never mix or harmonize. Sky, land, and sea all play different roles. For Riku and Kairi, especially, they should be the ones cooperating together since they directly interact with each other. Instead, their fate eventually leads to bigger clashes. Either land emerges through dried out sea, or sea engulfs the land out of turbulence. That's what represents Riku and Kairi. While Kairi is anguished and radicalized by pain and self-hatred, Riku is constantly paranoid to find order and stability.
I believe KH4 will eventually lead to those two fighting over Sora's fate. Their own guilt and finger-pointing will have them permanently break their bond and result in a huge clash. From there on, Riku and Kairi burn their bridges. They were never friends from the start. They secretly hated each other and only pretended to be friends because Sora was around. Without him, their reason for friendship diminished. Kairi blames Riku for never letting her do things while Riku blames Kairi for taking away Sora. They will break apart even more. With a huge fight. It takes few insults and fights to divide them for eternity.
As for Sora, sky above land and sea, hopelessly watches their difference and departure. No matter what he does or says, it doesn't matter. Riku and Kairi settled their goals. Kairi goes her own way to find strength just so she won't be labeled as "useless damsel", and Riku tries to reconstruct his world with Sora. But for Sora, his pain of losing his friends, on top of his guilt built up, is too much to bare. Even when he returns, things will never be the same. The damage is already done. Scales tipped for all three of them to fall down. Sora will become more desperate to hang onto friendships that never existed in the first place. His cry is ignored, his plead is unheard. Riku and Kairi go their own way out of hatred and despise.
The harsh truth is that Destiny Trio is doomed to break apart. Their friendship never existed. Their bonding broke down a long time ago. There's nothing for them to hold on any longer. As Kingdom Hearts progresses its Lost Master saga, their struggle will not only be limited to lost masters, but also include their own insecurity and instability. They're never meant to be happy. If anything, clock is ticking until they reach their Armageddon. Whether they settle their difference or break away forever, it's up to them.