It's a Party in Zexion's Room

“I wonder how it feels to visit your room after so long. The memories are rising into your mind now, aren’t they?”
The man’s voice resounded in the room, and Riku spun around.
A disgusting voice to go with these disgusting memories…
Riku frowned, answered in a low voice, “They’re not nice memories, unfortunately. I got this room from Maleficent, see.”
That’s right… Maleficent presented this room to me for my own use. Other than the time I spent aboard Hook’s ship when I went to get Kairi, I was in this castle. And, of all that time, I spent most of it in this room.
Swinging my blade—reading books—and if I had to say what else I did, I was just feeling irritated, thinking about things. Being angry about things.
I felt it was my fault that Kairi went into that state—but, then I’d wonder to myself if it wasn’t really a mistake to have left for outside worlds, and I would touch Kairi’s sleeping face gently.
“Seduced by the power of darkness, you lived in this room. You threw away your homeland, threw away your friends—threw away everything, and all you got was this tiny room.”
“Shut up!” Riku spat, and flew out of the room.
(Remembering gave him a headache, and Xion absorbed the memory of Zexion scolding him.)

It’s contradictory, saying that we are searching to seek what we lost ourselves, Zexion thought, recalling their former research. I don’t think our choice to research the Heartless and their creation process was a mistake. But as a consequence, we lost our own hearts.
“You really should have known that this was going to happen,” Zexion began, almost admonishingly. “Before arriving here, you travelled through many of the worlds of your memories. However, I’m sure all you met were beings of darkness. See, there’s nothing left in your heart but dark memories. The memories of your home—have vanished.”
“You’re lying! I remember everyone from the islands just fine! Tidus, Selphie, and Wakka too! Kairi! Sora! Everyone—My… my… important friends…”
Riku clenched his fists and looked down.
“And who was it who threw those friends away? Did you forget your own actions? You destroyed your own home!” Zexion shouted, and in that instant, darkness wrapped around them—and rain began to beat down on Riku. Thunder echoed in the sky—and then the world began to crumble away.
“The island where you were born, where you grew up, tore apart, crumbled away, and many hearts vanished into the darkness. It was your fault!”
Yes—if I hadn’t been so easily tempted—if I hadn’t believed his words—
“The one who got sick of the small-island lifestyle, thoughtlessly opened the door to darkness and destroyed the islands was you! You were tempted by the darkness. And now, you are a complete resident of the darkness.”
(Both Zexion and Ansem said this to him. What Zexion really means is: "I thoughtlessly opened the door to darkness and destroyed Radiant Garden. I was tempted by the darkness. Now I'm a resident of the darkness.")

I don’t know when I fainted. When I woke up, I was in this very quiet room. But it’s a kind of lonely room.Xion got up, and shook her head slowly. There was no-one in the room.
I must have—yeah, I collapsed in Castle Oblivion. I found out about myself in that castle. My true identity—
My…
I—wasn’t me. I wasn’t anyone. But, well then, who am I?
Then, Xion REMEMBERED where this place was.
This place—I know it, Hollow Bastion. The world where Riku and Sora fought. And, this is the little room that Riku used during the time he spent under Maleficent. I remember. I absolutely remember what happened the first day I came to this world. About Riku, about Sora, everything.
Xion got out of bed and left the room.
So, Xion—where do you think we should go from here?
(It was Zexion's former room - quiet, lonely, reading books, getting angry. Ansem might as well have been scolding him instead of Riku. He threw away everything, and this is what he got in return - nothingness. He regretted what they lost, like Riku, but he still didn't think it was a mistake.)