Bleach: Starting as a Vasto Lorde

A/N: Here's a reference for Kazuya's temporary gal form (Jingliu from Honkai Star Rail)

...

It required immense self-control for Kazuya to finally utilize the gender-changing feature of his System. Now, he could step into the biggest fight and use his Hollow powers without anyone being able to link him to his Shinigami identity. The only traits he retained from 'Kazuya' were a captivating voice and silver locks. Even his figure had shrunken down to 5'7".

The only concern left now was Aizen, who had utterly dominated the battle with Kisuke. The perverted shopkeeper had backup plans prepared for everything except for Aizen's powerful illusions.

As he contemplated, Yoruichi's Hollowification reached its climax. The slimy white matter drifted to her face and formed her distinct Hollow Mask. To no one's surprise, her Hollow mask took on the shape of a cat, complete with two sets of ears - one large and one small. Three claw-like marks etched above the hollow sockets for her eyes, while sharp canines filled her mouth with a fearsome appearance. 

{Yoruichi became… cooler.}

'Surprise surprise.'

Suddenly, a pair of blazing golden eyes ignited within the hollow sockets of her skull. With a deafening roar, she shattered her restraints and materialized in front of Aizen in an instant.

Kazuya was taken aback, unable to sense even a trace of spiritual energy behind her lightning-fast movements. She relied not on Shunpo or Sonído, but on her own body to achieve this speed. She had also completely suppressed her spiritual energy, blending seamlessly into her surroundings as if she were one with nature itself. It was a monstrous display of power and control that left everyone both impressed and terrified.

Aizen's piercing gaze widened in surprise as he saw Yoruichi's fierce white claws approaching his face. As impressive as she was, she was far from the level of successfully ambushing Aizen. Raising his katana in one hand, he casually blocked her flurry of offense. Her sharp claws could've torn any other Shinigami to shreds. Yet Aizen's fluid and elegant sword skills easily deflected the extreme force behind her attacks.

He made Hollowfied Yoruichi look weak, when in reality, she could floor your average Captain with ease. After all, she reached the realm of high-speed movement techniques with her body alone, making it extremely hard to track through a Shinigami's innate spiritual pressure perception. Her fighting style abandoned any martial arts and purely relied on her bestial instincts, which made her unpredictable.

Technically, she was one of the best fighters in the world, but she lacked the crucial element that separated good fighters from great — intellect. In a high-stakes battle, where quick thinking and predicting the opponent's next moves were paramount, her skillset fell quite short.

Aizen's lips curled upwards into a sly smile as he extended his left hand, fingers spread wide. With a flick of his wrist, a small orb appeared above his palm, pulsing with dark, otherworldly energy that seemed to bend and twist the air around it. "Hado #90, Kurohitsugi."

Caught in a powerful torrent of gravity, Yoruichi was pulled down to the earth. A surge of dark energy erupted, its surface etched with intricate symbols that glowed ominously. The black energy cracked and hummed before coalescing into a massive box-like structure that enveloped her.

Massive spears pierced through the black box, each going through Yoruichi and heavily wounding her.

For once, Kazuya was grateful for his blindfold. Otherwise, someone sharp like Aizen could've easily discerned his raw concern for Yoruichi, making his gender switch useless. The chantless version of Kurohitsugi could heavily wound a Captain-Class. It was only natural for him to feel worried over his loved one's safety.

{Calm, Partner… you can't act like you care for her. We must play the role to perfection.}

'I know. I know.'

As the black box vanished, Yoruichi's bloodied body came into view. The once graceful figure was now marred with deep wounds, but even as they watched, the flesh and skin began to knit itself back together. The newly regenerated skin was covered in the signature-white armor of Hollows, adding an extra layer of menace to her already dangerous appearance.

Yoruichi didn't seem bothered and dashed past Aizen. Her sharp claws extended eagerly towards his back. As a natural hunter, she couldn't not attack Aizen's 'vulnerable' spot. But just as she was about to make contact, a shimmering emerald shield materialized behind him, thwarting her mischievous assassination attempt and protecting him from harm.

Unable to attack his back, she loomed over Aizen then appeared on his right. Becoming a white blur, she moved in every direction, accelerating even beyond her previous limits. After having fun stalking her prey, she pounced at him with her claws open.

"Hiss."

Aizen, catching a glimpse of her figure in the corner of his eye, smiled before impaling her chest with his katana. "Tremendous raw speed and explosiveness. It's all wasted with a barbaric mind."

He pulled back his right fist and threw a punch at her face. Her Hollow mask shattered, revealing a pale-skinned Yoruichi.

"Bakudō #99, Kin."

At Kisuke's ominous whisper, thick tendrils of dark spiritual fabric swirled around Yoruichi, tightly binding her arms behind her back. Desperate to break free, she strained against the tight grip, but her efforts were in vain as heavy metal weights appeared out of thin air and pressed her body firmly into the ground below.

She couldn't move an inch.

Kisuke tipped his hat. "This is where I take my leave, Aizen-san. Thanks, and fuck you for everything~."

"Stop right there, Kisuke," Kazuya called out, his blunt voice, although soft, was just as if not more enchanting for the two men present at the scene. "Where is my Tier?"

He and Tier lived in the same town as Kisuke, so it'd be strange if he acted like they never ran into each other.

"I don't know. You should ask this man." Kisuke pointed an accusatory finger at Aizen. "He's responsible for everything that happened here."

Aizen's eyes widened momentarily before a hint of intrigue flashed in his eyes. "You are the Vasto Lorde with Tier Harribel… I was under the impression you were a man."

"Disappointed that a woman slew the King of Hollows?" Kazuya smirked and let his Reiatsu loose — immediately a Reiatsu almost as dense as a Captain filled the air, the kidō surrounding them creaking. "Scum of Soul Society. Where is my Tier? Don't make me beat the answer out of you."

Overbearing and authoritative — he tried to be consistent with the character Tier played.

Aizen chuckled. "The sheer arrogance… I'm amazed you didn't take the chance to rule Hueco Mundo. You'd have been a fine replacement for Barragan."

"Not interested in playing the ruler." Kazuya shook his head. "You leave me no choice. Kisuke, get out of here. You don't want to become a bystander casualty."

Kisuke spread his paper fan to cover his face. As he tapped his feet on the ground, an explosion of smoke filled the air, allowing Kisuke to retreat without leaving a trace.

Kazuya clenched his fist, swearing to beat the ever-living hell out of Aizen.