This takes place at the end of Kingdom Hearts III. After the final battle, the X-blade hasn't been destroyed and Sora have it with him. Now the battle is over. All of them are fine, except for Kairi.

- It's finally over! King Mickey exclaimed

Everyone nodded, except for Sora who shook his head is disagreement

- No

- We'll find Kairi, let's head back to Master Yen Sid, We can figure it out. Riku stated

- No, I know what to do

- Sora! Mickey exclaimed

- My whole journey began the day I lost her, and every time I find her, she slips away again. I thought we'd finally be together but she is out there, alone not for one more second. Sora answered determined

- We'll come with you, Donald retorted

- That's right, continue Goofy

- Thank you, Donald, Goofy, but this time, I have to go alone

- Sora, listen, the power of waking isn't to go chase hearts around. Even if you do find Kairi, you might never come home to us again. The king interrupted

- I will! And we'll both be back before you know it

- Please

Riku came closer to the king

- Let him go, Mickey

- Riku

- His hear and his mind are made up, now believe in him

The King finally nodded

- Safe journey Sora

- Thank you

And with that, he left he knew exactly what he had to do. And one more time, these words echoed in his head: May your heart be your guiding key.

He pointed the X-blade toward the sky, opening a passage which he went through. He went back to the realm of darkness, but he wasn't lost. As soon as he arrived, he felt her heart. He was following Kairi's heart's light, not though his eyes, but in his heart he could see it clear. He soon found her, she seemed lost.

- KAIRI! He ran toward her

- Sora! She opened her arms to hug him as he was coming close to her.

- I'm so glad to see you're okay!

- What happened? I don't remember anything, I woke up here, in the realm of darkness

- You were unconscious, Xehanort struck you with his keyblade to summon the X-blade, you were the thirteenth, that's why you couldn't use your keyblade anymore. I thought for a second you were gone, but I could feel in my heart that you were somewhere and that I had to go search for you.

- Does that mean that we're finally both gonna be okay together and come back home together?

- Yeah, but Kari could see something was wrong

- What's wrong Sora?

- Nothing, let's go home!

And though, he opened a portal thanks to the X-blade

- Go first, I'll be right behind you

- No, let's go together.

- O-okay

And so, they both walked toward the portal. As they were close to each other, Sora slipped a paper in Kairi's pocket without her noticing it.

As they were only a few steps away from the portal.

- Sorry Kairi, Sora whispered

- What for Sora?

But she didn't get an answer, and was pushed by Sora through the portal, which closed right after she crossed it. She needed a few minutes to figure out everything that just happened and for her eyes to get used to the light of this world.

Why had Sora stayed behind? She really didn't know. She realized she was on Destiny Island, she was home. That's when she heard someone calling.

- KAIRI!

- Riku!

She saw Riku running toward her, closely followed by the king, Donald, Goofy, Axel, Aqua, Ventus, Terra, Shion, Roxas and Namine

- How did you get back? Where's Sora?

- I was lost in the realm of darkness, he came thanks to the X-blade and opened a portal for us to come back, but as we were going to go through it together, he pushed me through the portal and then it closed behind me. I don't know what happened, she cried in Riku's arms.

That's when the King noticed a paper in the sand a few steps behind her, it must have fallen from her pocket while she was running toward them.

He took it and saw it was from Sora.

- Kairi, you had a letter from Sora in your pocket!

- He-he must have put it there while we were walking toward the portal. She said between sobs before reading the letter.

I'm sorry Kairi,

I'm sorry I couldn't come home with you, but after everything that happened, the X-blade hadn't enough power to bring us both back here. When you'll be reading this letter, you will probably be safe, home, with everyone. I know you might wonder why I did this, exchanging my place with yours, but I'm sure you can understand, I couldn't bear to live if you weren't happy, I couldn't leave you alone. As you're reading these words, the X-blade is probably destroyed, that means you should be able to summon your keyblade again.

She quickly tried, to be sure, and, yes, in fact, her Destiny's Embrace appeared in her hands.

As you crossed the portal, the lack of energy of the X-blade has also probably made it unstable and sealed definitely the realm of darkness. Now, all of the people I care about are fine, and the most important, you are okay. I hope that you will be happy, that's all that matters to me. I know I would have liked to be there with you to cheer you up, but I can't, so, be happy, even without me. I also hope you'll forgive me for not keeping my promise, I will not be able to come back to you this time.

And don't forget this, I love you, I always have and I always will, even if our bodies are fare away, I know our hearts are still connected, and this will never end.

Goodbye Kairi

Sora

- Idiot! Why can't you see I can neither live if you're alone there! She cried. We have to bring him back! We can't leave him alone in the darkness! I can't leave him alone…

- I'm sorry Kairi, but if Sora's right, the realm of darkness is sealed, away from our realm. And if it is the case, we won't be able to bring him back.

- No, no, no, no, no, NO! I refuse to believe it, there must be a way to bring him back, there has to be…

- We will try to bring him back Kairi, we will try as hard as we can, they all promised

On his side, Sora was sitting near the water, in the realm of darkness, at the same place he had been sitting with Riku a few years before. He suddenly felt a wave of sorrow and anger reach his heart.

- Kairi must have found my letter… I know I should have been with her, but I couldn't, I had to choose between me and her… And, I couldn't leave her alone, could I? He asked, to no one in particular, now he was the one alone. Maybe from here I can make sure that the darkness will not endanger the realm of light again, that all of them are safe now… Maybe, if the world is made of light and darkness, I'llbe the darkness.

After a few more days here, the darkness were starting to get to him. A part of him wanted to blame Kairi for his situation, wanted to blame everyone for leaving him alone. But he was fighting these thoughts hard. It wasn't their fault he was here. It was no one's fault. He had taken this decision all by himself and was at peace with it. He was wondering if maybe he'd be consumed by the darkness. Aqua hadn't completely been consumed, even though she stayed there for years, but she had hope to come back. He hadn't, he knew he would never go back to light. However, even if, in the end, he would probably be consumed by darkness, he would fight it as long as he could. He didn't want to be part of darkness. And so, each time he felt these thoughts coming back, he remembered why he had done it, remembered his friends were safe, Kairi was safe, and just thinking about it was enough to push the thoughts away.