Appears around late 20's early 30's
It is often said that pride comes before a fall and in the case of Sousuke Aizen it seems that this sin is the main, if not the only, cause of his failure.
At Aizen’s first appearance he seems a simple enough man to understand. Just another captain. Perhaps one who has a little unexplained conflict with some of his colleagues, one who is too soft on his lieutenant who is clearly not of much use to him in battle, but nothing much of interest. He even seems to be in agreement with Renji and other, seemingly more main characters, about the suspicion surrounding Rukia’s situation. If anything during the first few episodes in which he appears he can easily be almost forgotten, fading into the background as an unmemorable character among many new ones that have been introduced.
That is, of course, until his apparent murder.
This is where the tables turn for Aizen. It seems as though there would be no reason for this kind and bland man to be killed, by one of his own no less, yet there is no other foreseeable explanation. As suspicions grow and Soul Society on its head only days before Rukia’s execution, but by who?
When the revelation of the betrayal behind Aizen’s staged death come to light I believe that almost all of those either viewing or reading Bleach at the time were shocked to find out about Aizen’s hidden agenda, which just goes to show the complexity of his character. The warm smile used to keep others at his side is so easily turned to one of mocking and superiority as he turns his blade on his own vice-captain.
And this is only the tip of the iceberg as we uncover more of Aizen’s secrets and his desire for Rukia’s death, and the obtaining of the Hyougoku, next becomes apparent. The fact that he orchestrated the entire death sentence, as well as all the events surrounding it, without anyone else suspecting a thing until he reveals himself, only just begins to show how manipulative the Shinigami can be. All first impressions given by Aizen have now been upended and this is why I believe he is one of the most original and complex antagonists in the series, if not in any of the popular anime series that I’ve ever seen.
After this point Aizen continues to use his manipulative nature with his heavy influence he seems to hold over most everyone he comes into contact with to achieve his goals. Yet, surprisingly, he still manages to maintain his outwardly polite persona. Even keeping up mundane routines such as serving tea at each meeting he holds – even deeming this tradition more important than dealing with Ichigo and his group when they come to invade Hueco Mundo – and otherwise still acting as a respectable captain should.
Although this is where the cracks in Aizen’s plan start to form. As time goes by his attention is always kept on Kurosaki Ichigo, yet never is anything done about it. The boy’s powers never managing to do much more than hold his interest for small periods of time, yet in the end it will be these powers and the unexpected growth which they undergo which will be his downfall.
Another thing that has not changed since his time at Soul Society is his disregard for his subordinates. Many times he is shown to feel nothing but perhaps some annoyance at their incompetence any time any of the espada or other arrancar fail their missions, or even die. During his final battle with the gotei 13 he even disposes of Tier Harribel himself, noting that even the army he handpicked himself could not compare to his power level even when used as a combined force. This shows his vast superiority complex, his belief that there is nothing nor no one that could even begin to come close to his power level - this, again, making him overlook the growing skills of Ichigo and his pride never letting him consider that this human boy could ever surpass him.
Every move Aizen makes is cold and calculated, not a single one showing any remorse or compassion for those he effects. In his eyes there is only his final goal and those who fall through the corse of his actions are merely things that got in his way. In fact the only time Aizen manages to show any real emotion is during his final confrontation with Ichigo where he becomes enraged at his inability to understand the boy before him and the fact that anyone could get the upper hand over him.
Even post defeat it seems that Aizen’s air of superiority cannot be quelled, the sentence he receives for his actions bringing him to do nothing other than mock those sentencing him, earning himself a yet longer imprisonment. Also, even when help is offered in the form of the Vandenreich he turns down their offer to help free him, his pride still not allowing him to agree to work underneath anyone, whether it is his only chance of freedom or not.
All his existence it had been taught to him that this was a place to fear, or at least to be wary of, yet for the life of him he had never been able to understand why. Was it not alongside these warnings, these patronising rules lain out for students at the academy, that they were taught the inhabitants of this realm were the very things they existed to stamp out? Then what sense did it make to avoid their home? The very place they were off guard, not expecting attacks from anyone bar the sparse fights between their own kind.
The grains of bone white sand shifted beneath his feet as he crossed the barren land, the pitch black horizon broken only by the great marble structure that had become his current place of rest. He would not call it a home, if anything it was more like a prison. A place for him to wait for his own plans to yield him fruit, to wait for the actions of others when there was nothing more he detested than allowing anyone but himself to make each precise move in the game of chess he currently played.
Each step took him further into the shadow of his own palace, beneath walls which he knew held false sky, held his army. He knew because he had put it all their, maybe not with his own hands but with his will. There was not an inch of it that he did not know and there was not an inch he did not despise.
The whole thing had been designed with his every need in mind, yet even he did not account for the silence that seemed to have taken root in each empty corner. A silence that muffled out even the briefest of conversations he held with his ‘comrades’. It pressed heavily against his pores, searching for a way in but he knew it would not find it – even if somewhere, deep inside, he craved the respite it could give him from the constant ambush of thoughts that filled his mind. They clawed at each dark, twisted crevice of his insides; pushing him onwards, leaving him to forever strive for something more, something that would finally satisfy him.
The barest of sneers pulled at his lips, an ugly curve that marred his usually impassive features. A pleasant thought, if he could for an instant believe such a thing would ever be within his grasp. How he wished to be as moronic as those that had surrounded him for countless years, pacified by a broken system and never questioning the rules that bound them. Pathetic though they may be, it was a fulfilled life this ignorance brought them and though Aizen surely did pity them he envied them too, in equal parts.
By now he had become aware of the approaching reiatsu, weak even compared to the poor standard with which he was used to by now. He knew of her type, a lowly arrancar girl, but not her name. What did it matter? She was no more a person to him than a flea was on par with the dog it so desperately clang to for sustenance. Nevertheless, she had her uses, for now, and once he was gone from this place he could not care less what became of her or any other of her kind.
The pitch of her voice grated unfavourably against his ear drums, jarring with the silence he had become so accustomed too, yet none of his displeasure showed on his face. Instead, a simple, small smile lifted his lips; mahogany eyes warming as he turned to gaze upon her where she hovered at the palace’s large and only entrance.
“Yes, we shall now begin.”
Darkness was all that ever greeted him anymore.
Not that there had ever been any hope that anything else would reveal itself upon each new opening of his eyes. Yet still they opened; a force of habit after each period of rest. These were the only things with which he had to measure the time passing now, the small differences between states of consciousness. Though he was never truly asleep, never able to really rest.
His thoughts, they haunted him.
This was nothing new, not really. His mind had always been a double edged sword that constantly drew at any slim chance of happiness he had ever managed to possess, slaying it as it brought him yet more nagging voices speaking of dissatisfaction and distrust. How could others possibly live in a world so naïve, so pathetic? How could he stand by and watch them all rot, day in day out?
Although there was some change. These days his thoughts rarely drifted to the future, the plans for his release thought over and revised and perfected long ago, instead they dwelled in the past. Each swing of a blade, each raise of each fragile limb of each opponent etched in his mind to be replayed every second of every god forsaken day in this place.
He had failed.
He would laugh, had he not been gagged. Let his fingers fulfil that itching need to curl against the palm of his hand were they not bound to the arms of this mocking throne. What a foolish concept. His failure. The thought left a bitter taste in the back of his throat and he left it there. This feeling of shame in his own abilities would be what made sure it would not happen again.
So again, he relived it.
Each twisting turn of plotting, manipulating. Each painful moment he had spent pretending. He filed each second of each day as he searched. Searched for the moment in which he had gone wrong. And every time he came down to one conclusion.
His lips curled into a snarl, the movement minute and restricted as the face taunted him. How ironic that the birth he orchestrated would be the one that left him here. It was almost as if-
The hesitation of his blade, the flicker of his resolve. So minute, so insignificant. What was it about this… child that made him think even for a moment that he might possibly be better off if he-
Of course it was nothing. He had merely let himself underestimate the boy in his single-mindedness. It had been foolish but it was not a mistake he would make again.
One more flicker of eyelids, unnecessary cleansing of unused optic orbs, and once more he was greeted with near blinding darkness. Or..?
A chuckle erupted from his chest, vocal cords still lax in his immortal frame. Perhaps he had finally gone insane. In the never-ending chasm of his mind where each detail of his life had been recorded and filed away it would be easy for one to become lost, especially when trapped with nothing but his own bickering thoughts in the darkness for company. Nevertheless, perhaps he should welcome the change. It may at least provide some way to pass his entrapment a little faster.
“Hello,” Came his reply, not an instant too late. In his current state of mild amusement he did not even bother to question the form of the one who had greeted him. “May I ask where I am?”
“Iwakiri Academy, of course!”
How quaint. His head dipped slightly, corners of his mouth upturning as his senses expanded and he detected the presence of many familiar faces.
Aizen will retain all of his abilities post his defeat. (ie: Immortal, kido master, etc. however it will take some time for his zanpakutou to regenerate.)