Thinking back, Riku knows that he loved Sora long before meeting Kairi. As a stupid young boy, he loved him as stupid young boys do: aggressively, through taunts and challenges, through races and wrestles. It wasn't until Kairi appeared, a competition, that he realized what it was he felt for his friend. It wasn't until he saw the tenderness she showered on them both—Sora especially—that he realized deep down he wanted to give the other boy that same tenderness.
Thinking back, Riku wanted to smack his younger self. For being jealous, for being blind, for being stupid. There was never a reason to fear she might take him away. Never a reason to not realize Sora felt the same way. Never a reason to seek what lay beyond their Islands when everything he cared for most was already there. To his younger self, Kairi had been in the way, stealing Sora's attentions away from him. While he would jest about having feelings for her, they didn't truly come until later, until he saw her comatose state. It wasn't that he had ever hated her, in fact he liked her. Yet resentment had always lived deep in the bottom of his heart right up until he saw that defenseless state; the state he put her in.
It flipped a switch inside him. His resentment switched to Sora, seemingly lacked any worry. Because Riku loved fiercely, with a narrow focus, and he could not comprehend how Sora could lavish care and regard to so many, nearly equally. This is something he has since come to accept and understand now. Sora is caring per his nature, and to take any bit of it away would be to change Sora completely. But back then, seeing him fraternizing with Donald and Goofy felt like a stab in the back. He was fine, he was happy, even without either of them there. Even when Kairi was so helpless.
Much later, when Sora was asleep and Kairi was safe, he realized just how stupid he had been. Sora never abandoned either of them. He chose Kairi even when he couldn't remember her, and he had only forgotten her for Riku's own sake. It was then he realized how much better Sora was than him, and how much he needed to be protected. For as much as Kairi was defenseless, Sora faced greater danger from his own sacrificing nature. He gave too much to too many far too often.
That realization instilled Riku with a force he had never known. He was used to playing strong, to playing the hero, but deep down he had always known he was weak. Known that Sora was the real strength holding him together. If his strength needed his own strength, than Riku would just have to become it. That was why he chose to watch over him from a distance. Why he chose to sacrifice Roxas even knowing Sora might hate him for it. Why he jumped into Sora's dreamscape without hesitation.
Now that they were home and safe, it was easier to forget. To forget his own wrongdoing, his faults. It was easier to lose himself to Sora and Kairi instead, to the lights that shone on the darkness still resting inside him. He has yet to bring himself to tell them, but what keeps him up into the dead of night is not his past—at least not usually—but his future. He dreams of himself betraying their trust, of falling back to darkness, a deeper, thicker darkness. One he fears he will never escape. But each time he does they hold him, with such gentle care that bittersweet tears fall from his eyes and he can't help but hold them back, telling them silently how much he appreciates them. How much he needs them both.
Because as much as he didn't think he did when they were kids, he does. He needs Sora who isn't afraid of tough love, and he needs Kairi who isn't afraid to bare her entire soul to him if it will make him feel an ounce lighter. They are both pure shining light, but their hues are different and he takes them in differently. He and Sora still wrestle, still tease, but they also converse in deep conversations about what they have gone through, about what almost could have been. There is a shadow that settles over them every so often until Kairi comes and sweeps it away with her never-ending optimism, a trait Riku loves and longs for. But sometimes he needs someone who can share in his negativity. Someone to actually shoulder his burden instead of just brushing it to the side.
Sometimes he wonders if he does not love Kairi enough. If she knows. If just feeling the way he does hurts her. Sora's been etched in his heart longer, they've shared more memories, and they share a connection because of their travels together that she can never have. He wonders and squeezes her hand that much tighter, hoping that someday he can think of a way to make it up for her. To fill in the lost time, the past resentment. A way to tie their hearts closer together than they already are. Because no matter how hard he tries to make it otherwise, and how much he truly loves and needs them both, his love for Kairi is always secondary.