Kyouka Suigetsu (鏡花水月) roughly: "mirrored flower, reflected moon", is a derivation of a Chinese idiom for grasping for what is unreachable

A Broken Mirror

The room was dark and empty, save for a single shaft of light illuminating a space of floor just wide enough for two people to stand. The outline of a lone figure can be seen moving slowly through the gloom toward the circle cast by the unseen lamp. His steps are all that can be heard. Aizen Sosuke enters the circle, clad in his usual white overcoat and purple sash. He stops, and stands at ease with an unusual look of displeasure upon his face.

"What do you want?" He says to the darkness. His tone is even.

"It's been so long since we've talked, hasn't it? I was starting to think you'd forgotten about me." Another voice answers from the shadows, it's low pitch and smooth texture is a perfect match for Aizen's. It seems quite amused. The speaker emerges into the second half of the circle. It is the spitting image of Aizen himself, down to the finest detail, a malicious smirk is upon its features. As long as he had known it, Kyouka Suigetsu had always taken his form when it appeared to him.

"As if I ever could." Aizen allows a faint smile to cross his features, closing his eyes in seeming resignation to his zanpakutou's will. They stand for a moment in silence, facing each other, a would-be reflection.

"Oh, but you wish you could, don't you, Sosuke?" His doppleganger's question goes unanswered, as the stern look returns to Aizen's face.
"Your great plan seems to be progressing well." The apparition begins to slowly pace to the left, circling around his master.
"I've got to hand it to you, keeping all those creatures in line without my help is quite a feat. You've been learning..." Aizen gives no response, and shows no intention to. He only stares straight ahead.
"But your escape from Soul Society never could have happened without me." The speaker returns to Aizen's gaze.
"Isn't that ironic? Soul Society, corrupt to its very core, founded on stratum after stratum of lies, deceit, and betrayal, is so direly weakened by the thing that has kept it alive. You should be proud of gaining such poetic justice." He seems to grow more amused by the minute.
"But was that really justice? You got Tousen to come along easily enough. He knew as well as you do what you were aiming for, but is that really enough?" Aizen averts his gaze.
"You used me to deceive them all. The people who trusted you, not just as shinigami, but as friends, comrades, even lovers, you wounded all of them with the very thing that wounded you, the very thing the three of you are supposedly trying to destroy. Now that's poetic." The sword can hardly contain his glee, while Aizen seems to lower his head further, his eyes now in shadow.
"You know, I have it that Soul Society itself was founded on a betrayal, many ages back. Ideals are just illusions too, you know. Actions define the future. After what you've done, don't be surprised if your brave new world turns out to be just a reflection of the old one."

"Like you and me?"

"Ha, touche."

Aizen returns his gaze to his tormentor. While still not pleased with the circumstances, his countenance is one of steely determination. His double is still grinning, as if he could find no greater joy in the world.
"You talk as though it is entirely my fault that things turned out this way. But really, I'm just playing with the cards I've been dealt. Namely, you."

"Well, that is correct, but you forget that the two of us are here together for a reason. We were meant for each other, Sosuke. I found your soul because I could tell exactly what you want."

"Oh? And what would be this miraculous insight?" Aizen asks petulantly, confident that his double is just trying to unnerve him. He didn't really understand him. None of them did.

"You loved to deceive them, Aizen, every minute of it. You love the feeling of superiority you get from earning their trust falsely, gaining their praise and accolades without them even knowing a single thing about you. It's damn near an addiction isn't it, manipulation? Feeling a world apart from all the rest, alone, unchanging, and eternal. I can't really blame you."
The blade chuckles, as Aizen's expression grows steadily more stern. But suddenly, it changes completely.

"You know, you may be right. Perhaps after all these years I've grown a little too fond of toying with other people. We all have our weaknesses, after all." He looks at his double again, the expressions on each of their faces seemingly reversed from what they were a moment ago.
"I did not, however, indulge myself simply for the sake of doing so." Aizen's gaze becomes serious again. "I did use you, and my aptitude for manipulation so that the universe would change. So the old order would fall, and a better one might rise from it. I strive for something greater, while you simply sit in the shadows and mock. Perhaps this is why you are the horse, while I am the rider."

The angered expression seems to melt from his other's face almost as quickly as it came to it, replaced with mild exasperation.
"Spoken like a true demagogue, I shouldn't have expected anything less from you. But I guess I can't give you too much credit though... Your grand vision got a little hazy when it came to that little bitch of yours, didn't it?"
Aizen shows no reaction to the mention of Hinamori, his one-time subordinate.
"Hell, 'killing' her was just a show for Ichimaru. You missed her heart when you stuck me through her because you wanted her to see the real you. You couldn't help but indulge yourself then, cou-"

"Stop. Now." Aizen's face remains calm, but he has obviously become more than slightly upset with the newest change of subject. "This conversation has no purpose. Leave me, I have better things to do."
Silence persists as he turns away from the other, knowing the futility of trying to walk out of this place.

"...Are you still waiting for her?"

"That is not your concern."

Silence reigned again, as the two seemingly ran out of barbs for one another. Finally, Aizen begins to slowly walk out of the circle of light the way he had entered, sensing he was being allowed to leave. But before he can get far, he suddenly stops, and without turning, asks.

"Why did you bring me here?"

His weapon, who had remained calm and quiet a moment before, burst out laughing.
"Someone has to keep your ego under control, you know. Pride always comes before the fall, and I'm not ready for you to fall yet."

Aizen continues to walk without another word. Eventually he leaves the inner world the two of them share, returning to the world of reality, and death. Standing alone under the single shaft of light, Kyouka Suigetsu wonders if he should have taken up that offer Shinsou gave him to trade users.

Author's Notes: I always assumed Aizen had better reasons for doing what he's doing than just being "the bad guy." There's plenty of evidence that Soul Society is really a pretty corrupt, oppressive place. So it's easy to imagine that Aizen and the others are fighting because of some wrong done to them and/or everyone by it. That's the assumption under which I wrote this, at least.