Even if Captain-Commander Yamamoto had been at his peak, he likely couldn't have killed Sōsuke Aizen in his current state. He might have even inadvertently triggered further evolutions.
"Captain Aizen, allow me to deal with these… remnants."
Gin Ichimaru appeared behind Aizen just as he was about to address the assembled Shinigami. He moved to Aizen's right side and placed a hand on his raised Zanpakutō. A sense of clarity washed over Gin.
He had long known the secret to resisting Kyōka Suigetsu's hypnosis: touching its blade. Direct contact with the Zanpakutō itself negated its illusionary effects. This explained why the captains, after being struck by Aizen, could briefly locate him. The moment their bodies made contact with Kyōka Suigetsu, their senses were freed. However, the effect was temporary. Once contact was broken, they were susceptible to the hypnosis again.
Gin pressed down on Kyōka Suigetsu with his left hand, drawing his own Zanpakutō with his right. Aizen, seeing Gin's actions, wasn't surprised. Gin had never hidden his desire to kill him.
Aizen, in turn, had always been curious about when and how Gin would make his move. For over a century, Gin had followed him, his intentions clear, yet he had never acted. Now, confident in his own power, Aizen dismissed Gin's contact with Kyōka Suigetsu as inconsequential. He assumed Gin was merely trying to protect the remaining Shinigami.
"Very well, Gin. I entrust these… pests… to you."
As the words left Aizen's mouth, a flash of white light erupted from Gin's sleeve, piercing Aizen's chest.
The assembled Shinigami watched in stunned silence. They had seen Gin draw his Zanpakutō, its tip aimed at Aizen's back, before releasing its Shikai.
Shinsō, Gin's Zanpakutō, was renowned for its speed, its extension and retraction faster than any other blade in Soul Society. In close combat, one had to constantly watch its movements, lest they be killed in an instant. Now, with Gin less than two meters from Aizen, his Zanpakutō aimed directly at his unguarded back, Aizen had been struck before he could even react.
"Ugh…" Aizen looked down at the blade protruding from his chest, then smiled. "Gin, I've always wondered when you would make your move. But I never imagined it would be now. Aren't you afraid this might be an illusion? That you've struck someone you wouldn't want to harm?"
Despite the gaping wound, Aizen showed no signs of distress. Yamamoto's Ryūjin Jakka hadn't been able to kill him. A mere puncture wound was insignificant. What truly puzzled him was the conviction he had sensed in Gin's attack. Ever since Kyōka Suigetsu's ability had been revealed, everyone who fought him, even Yamamoto, had hesitated, fearing they might be striking an ally under the influence of his hypnosis. Gin, however, had shown no such hesitation. He had been completely certain of his target.
"Of course, Captain Aizen. I wouldn't attack you without being absolutely certain I wasn't under your hypnosis." Gin raised his left hand, still gripping Kyōka Suigetsu. "I've been by your side for over a century, searching for a way to counter Kyōka Suigetsu. Perhaps even you haven't noticed… Everyone affected by your hypnosis can briefly locate you after being struck by your blade. After years of observation, I finally understood. Contact with Kyōka Suigetsu, regardless of whether its ability is active, negates its effects. Of course, this weakness is… practically useless against you. The contact lasts only for an instant. But, Captain Aizen… haven't you noticed? My hand hasn't left your Zanpakutō since I appeared."
"Gin, your observation skills are… impressive." Even with his secret revealed, Aizen remained calm. "However, you're mistaken. It's not that I hadn't noticed this… quirk… It's that I don't consider it a weakness."
He paused, the image of a man flashed through his mind: Sōya Azashiro, the eighth Kenpachi, whose Zanpakutō ability had been a perfect counter to Kyōka Suigetsu. Azashiro's presence had forced Aizen to use Shinji Hirako as a shield, to avoid detection. Only after Azashiro, believing he had misinterpreted the Soul King's will, had surrendered to the Central 46 and been imprisoned, had Aizen begun his research into souls.
Azashiro had been the most formidable, the most unpredictable opponent Aizen had ever encountered. It was Azashiro who had shattered Aizen's reliance on Kyōka Suigetsu, leading him to a fundamental truth about Shinigami combat:
"A Shinigami's battle is a battle of spiritual pressure. Sufficient spiritual pressure can negate, even erase, any Zanpakutō's ability."
This was why the Hōgyoku, responding to Aizen's desire, hadn't granted him new abilities, but had instead amplified his spiritual pressure.
"But Gin, even if you're immune to my hypnosis, a mere scratch like this… I can heal it in an instant."
The white light faded, Shinsō reverting to its sealed state.
"Indeed. If that were all, my attack would be… insignificant." Gin's lips stretched into a wide grin. "But Captain Aizen, do you know my Zanpakutō's true ability?"
"Hmm?" A flicker of unease crossed Aizen's face.
"Gin, isn't your Bankai simply an extension and retraction at five hundred times the speed of sound?"
"My, my. To think you believed such a blatant fabrication, Captain Aizen…" Gin's grin widened.
"But I'm afraid I must correct you. Shinsō's Bankai isn't simply an increase in speed. It's… something else entirely."
"Oh?"
"Upon release, Shinsō's blade turns to dust, piercing anything in its path. The dust contains a potent cellular toxin. While we've been talking, it has spread throughout your body, hasn't it?"
Fear, for the first time, appeared in Aizen's eyes. "Gin, you…"
Just when Aizen was about to say something, Gin's left hand released Kyōka Suigetsu then placed his hand on Aizen chest, and chanted in a low voice:
"Bankai – Kamishini no Yari!"
BOOM!
In an instant, massive holes appeared across Aizen's body. All the cells in his body that were made up of spirit particles were completely dissolved by the power of Gin's Zanpakutō, revealing the Hōgyoku floating within.
Gin reached out and grasped the gem.
In the setting sun, Aizen's body completely disintegrated.
"I have it! The piece of Rangiku's soul you stole."
Clutching the Hōgyoku, Gin gasped for breath. Despite his calm demeanor, the pressure he had endured during his betrayal, the weight of his actions, now lifted, left him breathless.
He had waited for this moment for over a century. To kill Aizen, to reclaim what he had taken from Rangiku Matsumoto, to avenge her.
He looked up at Rangiku, ready to face her, to return what was hers, to pledge his life to her protection.
But before he could speak, he saw the horror in her eyes, the desperate plea in her voice.
"Gin! Run!"
"What?"
Before her words reached him, he sensed something was wrong.
BOOM!
An immense spiritual pressure erupted behind him. He turned to see Aizen, fully regenerated and evolved. A third eye pulsed in the center of his forehead, his cocoon shattered, two pairs of butterfly-like wings sprouting from his back.
Gin's eyes widened in disbelief.
"How… is this possible?"
…
It can be said that Kyoraku and others witnessed Gin's counterattack on Aizen most vividly.
Even injured by Aizen, they hadn't imagined the secret to countering Kyōka Suigetsu lay in touching its blade. They listened to the exchange between Gin and Aizen, understanding the complex dynamic between them.
Gin, driven by an unknown motive, had followed Aizen with the sole purpose of killing him. Aizen, aware of Gin's intentions, had allowed it, confident in his own power, curious about Gin's methods.
Like a snake, Gin had bided his time for over a century, finally sinking his fangs into his prey, injecting his venom.
But to their horror, Gin had released Kyōka Suigetsu at the last moment. The cellular toxin, which he claimed had spread throughout Aizen's body, had only dissolved his lower half. His upper body, including his head and the Hōgyoku within, remained intact.