"M-Mr. Assassin..."
Beast III's compassionate smile instantly froze. As if sensing something inconceivable, her eyes constricted sharply.
"M-Mr. Assassin...you're right. Finding happiness in an illusion only helps me - it does nothing for others I have failed or those I still owe apologies. Suffering continues to exist..." A frail feminine voice echoed from within Beast III's body.
Impossible!
Beast III's eyes narrowed as all pretense of compassion vanished, leaving only naked disbelief on her face.
Because—
She had awoken!
"I see, so that was it." Shirou smiled at Beast III as he watched her confidence crumble. "Turns out you hid her in paradise as well!"
"Of course. Even for 'me', there is a world I desire, an outcome I wish for."
"No." he shook his head. "What you buried in that false paradise was your own innocence."
At his words, Beast III reacted as if stabbed in an old wound. Her face devoid of compassion, her smile faded gradually, her gaze turning icy cold as she looked down on him.
"Looks like I touched a sore spot."
He met her cold stare and smiled instead.
"I-I remember everything now. I remember what I did...I don't want to stay here anymore...let me out, let me out..."
Kiara Sessyoin's feeble pleas continued echoing from within Beast III's body.
"Let me out...I don't... I don't want to stay here anymore!"
Beast III lowered her head slightly and said in a low voice, "Quiet!"
...
She remembered.
Trapped within Beast III's body and lost in paradise, Kiara Sessyoin remembered everything.
She recalled her entire lifetime's worth of memories as a human.
She was not a person of this world, but someone from another world entirely.
In that world, after the great collapse that occurred in the 1970s, magi accepted their demise and introduced the once forbidden modern science. Eventually, they redefined the concept of a "soul" - what they dubbed the Spiritron, the outlet for the conscious mind's output.
In the past, the mind could only manifest through a flesh and blood body. However, magi succeeded in locating the "soul" and projecting its output port upon digital avatars in cyberspace.
As such, magi in that world who mastered this art were known as Cyber Ghosts.
Yes, it was the world of Fate/Extra CCC that Shirou knew of.
In Fate/Extra CCC, she was the daughter of a leader of the Shingon Tachikawa Eiten School. She was born and raised deep in the mountains, suffering from a terminal illness since childhood that foretold her death before age fourteen. Believers around her only pitied her and lent no helping hand, leading her to conclude that "people do not save other people." She even doubted whether the "humans" described in books referred to anyone other than herself.
Her father had always restricted her from secular contact. At the time, her sole pillar of support was the Hans Christian Andersen fairy tale books that visiting believers gifted to her as presents. It was not until she turned fourteen in the springtime that, enticed by those believers, she connected to the outside world's knowledge through the internet, curing herself of her previously incurable disease.
Once she became free to act and travel, the memories of those fairy tales faded to the back of her mind.
At the same time, her talents as a Cyber Ghost bloomed - she gradually reformed Tachikawa Eiten School, transforming it into a new organization of the magic world suited for the networked society. She rapidly increased the number of believers. Eventually, she violated all three taboos of the Tachikawa School, sparking internal power struggles within the cult that led to believers massacring one another until the entire school was wiped out.
And she herself was killed by the maddened believers who sought to monopolize her.
Yet she had developed her own digital conversion art, converting herself into a pseudo-Spiritron and transferring what remained of her soul into cyberspace. She entered the supercomputer found on the Moon - thirty kilometers wide and known as the Moon Cell, which served as the prison for the White Titan Altera who had invaded Earth in the past.
By absorbing the Moon Cell's authority from the Fate/Extra CCC world, Kiara Sessyoin became Beast III.
However, it seemed that because she was still mid-process assimilating and converting the Moon Cell's power into her own, a transfer anomaly connected her to another world, ripping her out of her own reality mid-transformation. Forcibly aborting her absorption of the Moon Cell's full power, she arrived in this world instead. Thus, her incomplete form of Beast III barely encompassed half the original power.
Most importantly—
Because of that forced transfer termination during her transformation, Kiara's innocent self was born!
Or to put it another way - this was the fundamentally [Good] aspect within Beast III that she derided yet preserved out of mockery. But due to the interruption cutting her alteration short, it ended up partitioned away from the main personality.
Initially, Beast III herself didn't notice this.
It was only after she was summoned into the Holy Grail War of this world as a Master and successfully established a contract with the similarly evil Red Caster.
Caster then planted the markings of an evil god upon her in an attempt to use her as the vessel for an evil god's descent.
Of course, Beast III successfully resisted the possession in the end. And this in turn was why the evil god moved to interfere with Kotomine, as the body he chose to descend into resisted him.
However, due to that disruption, the innocent Kiara lodged within Beast III escaped outside.
Beast III loathed this [Good] aspect of herself, the same way humans detested [Evil].
Indeed - Beast III, who took pleasure from malice and hostility directed her way, who forgave any and all affronts perpetrated against her, the only, only thing she couldn't tolerate was this [Good] self that had been split apart!
Unable to eliminate her innocent self and powerless to reintegrate with each other, Beast III could only imprison Kiara Sessyoin deep within her body, trapping her in paradise.
She assumed that this [Good] aspect would remain forever lost in paradise, but now, due to Shirou's words, she had regained awareness.
Awakened from blissful oblivion, Kiara began struggling, fighting to break out from Beast III's body!
"Behave yourself!" Beast III chided.
Gazing up at her, he chuckled, "Looks like even the Beast hailed as the [Third Heretical Pleasure Heaven of Deeply Grateful Existence] still has her own troubles."
Beast III stared coldly at him
"Let me out, let me out! You destroyed the school my father founded, leading all who believed in you to ruin! This is unjust, the Buddha will punish you for this! You are no Buddha, no Bodhisattva - you are a Devil!" Kiara's cries continued echoing from within the Beast.
Beast III was no longer gently smiling. Clutching her stomach with knitted brows, her expression conveyed disgust and distress.
Seeing that, he shouted, "That's right, Kiara! As I said before - villains don't heed the words of the good guys. But you did say you would keep trying. Yes, just like that!"
Kiara Sessyoin's struggles intensified, while Beast III's frown only deepened.
The pretense of compassion had vanished. Baring an expression of utter revulsion, she glared murderously at him. "Shut up!"