I always wondered if Sora was aware that he was a heartless until the start of KH2, and what his opinion was. This is just a little story to get people's creative juices flowing, and to entertain.

I in no way own Kingdom hearts or any associated media. Kingdom hearts belongs to Disney, Squeenix, and Tetsuya Nomura.

Everyone has a secret. Some are the innocent kind, like ice cream stolen from the freezer, or flowers pulled up instead of weeds. Then, there are the serious kind, like crimes, or horrible lies. There are passionate secrets, meant for no other eyes, and there are painful ones, locked away forevermore.

Sora wasn't sure what his secret qualified as. It was dangerous, yes, but he wasn't afraid of that. He knew that his friends would accept him as he was, That was the kind of people they were. But still, the fact of the matter remained. He knew that some of them wondered, as he had. Had, until he remembered.

That was when he first felt the fear, the fear of being rejected. But the moment passed, and he let the memory fade. The next time he felt the fear, it was mixed with relief. What he was, he was no longer. Roxas had seen to that. But still, the secret persisted. A secret great and terrible, one he could never tell.

It was the keyblade that gave him stability. Sora only had to reach for its warm light, and be reassured. What he was had no meaning to the keyblade. It measured him by thought and by deed, not by substance. At times, he was honored. At other times, he felt deepest shame, and unworthiness. But still, the keyblade refused to leave him.

At times, Riku and Kairi puzzled over part of his secret. He puzzled over it with them, laughing, and pointing them the wrong way. But he always worried what would happen, if they put the pieces together.

If they discovered the dark truth of the Antiform. The dark nature that Sora had endured, and accepted.

Sora feared Xehanort's Heartless. Not its strength, or even its presence. He feared its nature. Xehanort's Heartless was one of two heartless with human appearance and reason. The Organization XIII of heartless. And so Sora feared. Feared what his comrades, or even people that he had met afterwards, would say if they learned.

One of two... Xehanort's heartless was one of them. And Sora knew, without a doubt, taht he was the other.

At the time, he hadn't known about nobodies, hadn't known that a piece of himself was missing. So he carried on with his life, ignoring the signs. Ignoring the strength with which he felt his emotions, ignoring how he got angry or frustrated more easily. They say ignorance is bliss. A phrase Sora believed in whole heartedly. If he had known, he would have been more aware of his instincts, and could have become a second Ansem.
But he didn't. He was ignorant, and thus safe. It wasn't until he first accessed antiform that he knew. The familiar feeling of being cocooned in darkness, and the senses that came with it returned full force. He was scared, scared of how familiar the sensation was. He had been a shadow for but a few minutes.
Then he remembered Organization XIII, and the nobodies. The despair coursed over him like a wave, empowering him with a rush of darkness. He had never truly ceased to be a heartless. And without his nobody, he would always be a heartless. The antiform was proof of that.

But when the Organization began calling him Roxas, Sora relaxed. If the organization thought that he was Roxas, it meant his nobody had done something, anything, that meant he was no longer a heartless.

Riku confirmed it in the Castle That Never Was. Sora wasn't a heartless. When Roxas rejoined Sora, Sora was completed, and no longer a heartless.

But even complete, Sora was afraid. If he and Roxas ever separated again, only he would know. And so, Sora was afraid. Afraid that if Riku learned, he would try to help, and kill Sora, just as any other heartless. Donald would very likely act regardless, and think after.

So he kept his secret, hid the knowledge of his year as a heartless, and grinned. Some secrets, he knew, can never be told.