He always wished he was real.
If he was real, he could have been happy, being beside Naminé and beside Sora, talking about various things whether it be about the future or the past.
But he wasn't. These memories weren't his own, his appearance not truly his, his strength a distant imitation of another's. Even his heart wasn't real.
Because he was fake.
He hated being fake. It made him feel empty, like a useless existence in the world. But if he was fake, did he really exist at all?
Axel always made him feel like he did.
After he had left Naminé and Sora, Axel had come up to him, offering him comfort and truth. He would whisper sweet words into his ear, hold him close and kiss him, treating him in a way that made him feel real, made him feel needed.
But he was fake, and he knew that.
He was a replacement for the real Riku. The Riku who was supposed to be afraid of the darkness, Sora's real best friend, the one he knew Axel truly desired. And he couldn't understand why, but deep down in his fake-heart he knew that he could never take Riku's place, no matter how much he wished to.
Riku had called him fake. And although his memories were distant and distorted, no longer what they should have been, he could vaguely recall his former encounter with Riku. He had been created for…something, something he couldn't remember, but when Riku had called him fake, he could remember the false envy he felt, bordering on imaginary hatred.
So, he had absorbed Zexion. To be someone different, not just a copy of Riku, but to be his own person. He had taken Zexion's power for his own, but it hadn't changed anything. He was still just a fake, still so hollow and empty, still just a mere replica of Riku.
He despised his own not-quite existence, just as much as he despised Riku.
And even now, as he left the warmth and security of Axel's arms, left his sleeping lover behind to seek out his real counterpart, he could still feel the pain of his false-heart's yearning for Axel.
He would fight Riku, and he would kill him.
And if not, he would die trying, happy knowing that his false existence could finally end.