Now, I don't know Kingdom Hearts very well at all, but I love your writing and want to give this a read. So, I'm going at this without knowing, well, pretty much any canon.

It's oddly hard to not hate Kairi, even though it's very much obvious that Riku isn't looking at things with the most... stable eyes. His jealousy is just so vibrant. It's become a living thing, and it's just eating him.

but instead created fantastical worlds where he and Sora could be together and Sora would ask after Kairi, and her place in Riku’s imaginings This hurts. That Sora apparently has No Idea At All and, consequently, rips at Riku's heart again and again without knowing what he's doing.

Riku’s hands weren’t content to stay still, had to slip under the fabric of Sora’s nightshirt, had to taste the skin there, there and everywhere, had to trace the path of his ribs, to brush over young nipples, quickly growing impatient and stirred by the delightful way that Sora squirmed against him Eeeep. So wrong, yet so utterly sensual.

Held it in his heart, let it consume him, let it replace an important part of him, a part that felt shame at his actions, a part he no longer needed. This is so dark. Powerful. Wow.