Masks were dangerous. They had to be wielded carefully, or they may take a life of their own.
That was what happened to him. It was not that he used a mask; he was the mask.
For years, he was known as Captain Aizen Sosuke, the kind and strong leader of the Fifth Division. He was a man devoted to his soldiers, and caring to all underneath his protection. A model of what it meant to be a captain in the Gotei 13.
But that man did not exist… or so the real Aizen Sosuke said.
If one wears a mask long enough, then the bearer's face might come to match it. But while that did not happen in this case, the mask did indeed take a life of its own. When Aizen revealed his true heart to Soul Society, the mask he thought he had discarded remained alive inside the mastermind's inner world.
The mask could not tell when he was truly "born." In the beginning, he truly was a lifeless disguise, used by Aizen to hide his true intentions from Soul Society. But the deceiver did too good a job in making his disguise. Somewhere along the line, the mask was not too sure when, he became alive. While he could not tell when his memories started becoming truly his own (he inherited all of Aizen's memories, so that made recalling his life a bit confusing), he could definitely consider the day Aizen betrayed Soul Society as the day he truly became an independent soul.
More specifically, it was the moment the real Aizen stabbed Hinamori.
Conscious of the fact that he had been discarded and furious over the harm done to his valued lieutenant and friend, the mask fought with blind fury against his creator, but to no avail. There was not so much of a twitch of ground conceded to the newborn soul, and the mask could do nothing but watch helplessly as Aizen cut down Hitsugaya, then Abarai, and Kurosaki, and finally retrieved the Hogyoku from Kuchiki Rukia. His shouts of warning to Shihoin and Soifon to finish off Aizen while they had him vulnerable went unheard, and the mask cursed when he saw that his maker had escaped unscathed from Soul Society.
Subsequent battles for control of Aizen's body all ended like his first attack. Despite being born from his vast power, the mask was still outnumbered two to one. Still, even without Kyoka Suigetsu at her master's side, the mask doubted he could have defeated Aizen alone.
So he tried a number of ways to convince Kyoka Suigetsu to turn against her master. First he attempted to reason and to persuade her that it was in her best interest to fight Aizen. She did not give so much as an inch.
"Sosuke is my bearer, not you," she said to him, "I don't know how a mask like you came to exist, but you are not my master. You are merely an anomaly."
He then tried subtler means of manipulation. They all ended in failure. After all, Kyoka Suigetsu had been with the master of manipulation far longer than he had, not to mention the fact that she was a caster of illusions.
But a chink appeared in her armor. Aizen's increasing obsession to have the Hogyoku as his source of power gradually began to upset her, disrupting the previously flawless relationship between the two. It was minor annoyance at first, but over the next couple of months it grew into jealousy.
Unfortunately, Aizen's move to make the King's Key came far too soon for the mask to take advantage of the zanpakuto's jealousy, and she continued to fight as her wielder's weapon. She hoped that if she proved herself as a worthy weapon during the battle, he would continue to rely on her power instead of the Hogyoku. It was for this reason that Kyoka Suigetsu fought by her master's side, when the man who would be god attacked Karakura.
As the captains and Visored attacked Aizen, the mask fought his maker with greater urgency. He knew that he would not be able to win, but he hoped that he would at least distract his enemies long enough to leave an opening. Again, these efforts were in vain, and he watched in horror as Kyoka Suigetsu painted her illusion and caused Hinamori to not only be stabbed again, but also be impaled by Hitsugaya.
The mask knew he should not waste his energy on futile attacks, but the sight of this cruel action set his fury ablaze once again. Predictably, this rage went almost unnoticed by Aizen as he cut down the rest of the captains.
Even though the mask knew that Aizen had planned ahead, the appearance of Yamamoto sparked hope. Of all the individuals Aizen had made enemies of, the captain-commander of the Gotei 13 was the only one who was actually Aizen's superior in sheer power. But this hope was cut short as the Arrancar Wonderweiss engaged the old general. While Yamamoto defeated his opponent easily, the failsafe placed in the Arrancar brought the captain-commander down with his own flames. Yamamoto's last attempt at Kido did catch Aizen off guard, but it did little more than singe the man.
As Aizen boasted to Kurosaki Ichigo on how the former had manipulated the events that caused the latter to become a shinigami, the mask fervently hoped that Aizen's plan to make Kurosaki stronger would backfire disastrously on him.
When Urahara appeared on the scene (along with Kurosaki Isshin and Shihoin), the mask once again raised his hopes. Urahara, although not as strong as Aizen, was the only person the evil mastermind considered his intellectual equal, if not superior. In this case, the mask hoped that brain would win out against brawn.
Unfortunately, that would not be the outcome this time. For Aizen was now fully exploiting the Hogyoku's power, and using it to evolve beyond his shinigami form into something new and powerful.
Aizen's merging with the Hogyoku and the subsequent metamorphosis he underwent was both a blessing and a curse. On the one hand, Aizen's discarding of his Kyoka Suigetsu in favor of the orb was the final blow that severed the bond between the former shinigami and his zanpakuto. Not only did she disavow any loyalty to her wielder, she also became motivated to take revenge on him.
It was a move that the mask had been hoping for, but it was too late to do any good during the battle. Even though he had Kyoka Suigetsu aiding him, they could not overcome Aizen, now that he had the power of the Hogyoku.
With the Hogyoku awakened, Aizen could now fully exploit its abilities and grow to become the godly being he wished to be.
Then Ichimaru Gin struck.
In theory, Ichimaru's betrayal was not that much of a surprise. But the precise moment and timing of his attack had been impeccable, almost perfect. Almost. While the mask lauded the fox's effort, he knew it was in vain. For it was not Kyoka Suigetsu that was helping Aizen now, but the Hogyoku. If it were not for that cursed orb, Ichimaru would have succeeded in killing Aizen.
But still, at that precise moment when Aizen lay at the brink of death, just before the Hogyoku revived him and increased his level of power again, it allowed a window of opportunity for the mask and Kyoka Suigetsu.
They went into Aizen's mind, and, put simply, trashed the place.
They had only a short time, and they were quickly forced out by the Hogyoku once Aizen had been revived, but the damage was done. The man who had once been a master of manipulation and strategy would not notice, but his intellect was now not quite as keen as it once was. In light of Aizen's near-divine power, it might not mean much, but it was all they could do.
Then Kurosaki Ichigo arrived.
At first, the mask was worried that the half-shinigami had only come to get himself killed, but then he saw something that Aizen would have, had the latter not denounced his katana. If the newly evolved traitor had taken time to use his eyes, he would have seen what both the mask and Kyoka Suigetsu saw in Kurosaki's eyes: strength. Strength that promised, no, guaranteed Aizen's defeat.
And both of them had their observations about the substitute shinigami confirmed when he forcibly moved Aizen to a battleground free from the danger of collateral damage.
If Aizen had used his brain, even the one that had been damaged by the mask and Kyoka Suigetsu, he would have realized he was now in a fight that he could not win. But he was drunk on his newfound power, and refused to accept reality.
The mask had been so elated when Kurosaki launched his Final Getsuga Tensho. The mask, who still identified himself as Captain Aizen Sosuke of the Fifth Division, was ready to lay down his life to ensure that the monster he cohabited a mind with would die.
The horror of realizing that he and his creator were still alive quickly turned to relief after it was revealed that Urahara had a plan. While Kurosaki's Mugetsu had not killed Aizen, it had shown the Hogyoku that its bearer was not the most powerful being present. That left an opening for the mask to encourage the orb to abandon its master. It did so without hesitation.
And without the orb to help him, Aizen was trapped in Urahara's seal.
At long last, the traitor Aizen was defeated.
Presently, he was sitting alone in a cell, now that the Hogyoku had discarded him, and Kyoka Suigetsu was separated from him.
Well, Aizen was not completely alone.
The mask was his only company. And the mask made sure that should Aizen ever be let out of the Muken, that he would be incapable of harming anyone ever again. After all, he had a few millennia to mold the former manipulator's mind, and the damage caused by he and Kyoka Suigetsu now evened the odds.
The mask knew that even if he won control of Aizen's body, nothing would change externally. He would still be trapped within the Muken, and he would still be sentenced to twenty thousand years in solitary confinement. After all, it wasn't as if he could ask for parole, claiming to be a new personality of Aizen; no one would believe that tale, and with good reason. No, any chance he had at being captain in the Gotei 13 died when Aizen stabbed Hinamori in the quarters of the Central Forty-Six. It was a cruel irony, that the moment he was born was the moment he lost any chance of attaining his desires. Still, the mask would not allow himself to be embittered by this fact. He decided that he was content in being the unknown warden of Aizen, the first line of defense in making sure the traitor stayed in his cell.
The traitor Aizen Sosuke was wrong; the Captain Aizen Sosuke that Soul Society once knew did exist, if only in the traitor's inner world. He may only be a mask, but he existed nevertheless. He could not tell when he was born, only that he was born of Aizen's deception and Kyoka Suigetsu's illusion. While neither were ideal parents, it was enough for the mask. He was born, he existed, and as a captain of Soul Society, the mask of Aizen Sosuke would fight the monster that created him.