she's not gone at all, merely vanished from your world
Fiora, full of ball joints and crosslatched platepieces, couldn't remember her prior self. She was a mock-up, a half-cock, an absolute mess of inhuman engineering.
She didn't like it. In fact, she hated it. She could stand as defiantly as she wanted at the head of the party and swivel side to side, step to step, on and on and on and on, flexing dual blades and the promise of a drone flight all in glittering rainbowchrome armor, but it wouldn't change how she felt inside.
Inside...with Meyneth.
She was still human, in the only way that really mattered - which is to say, the Machina were human too, thinking and feeling and loving and breathing and knowing - but she shared her body, her very life force, with the goddess who had created the beings who had allowed her to keep living to this day.
All life was born from and would be returned to Zanza, eventually. They all understood that. But Fiora's life...?
It belonged to Meyneth. Not Lady Meyneth, no, just...Meyneth.
Meyneth was eternal, and Fiora was eternally young. She fought with her power, with her spirit, with her will to restore peace and dismiss this violent revolution of Egil's from the world. Yes, Fiora was a spunky one, and she'd always make the best of any situation she was handed, but as all good things do, eventually - inevitably - that endless flow of love and power came to an end.
It was because of Zanza. As all things were. But inevitabilities don't make mourning for what feels like your own soul hurt any less.
It wasn't until after Meyneth had gone, then, that Fiora truly realized all of this. Realized how crucial the second occupant of the hulking, clanking frame was to every bit of soaring energy she channeled as she fought, as she lived, as she kept herself from dying.
Alone, strength fading every day, Linada's genial advances turned away as cordially as possible, Fiora wept for what she had lost, not because she had lost it but because Meyneth herself had had to lose it. After all she had given...oh, it wasn't fair.
Vanea happened upon her lingering by Junks, one night when all the stars seemed to mourn as well. The hand clutching futilely at the prismic crest of the Lady's Monado...
She could have asked something so simple and straightforward as "Are you alright, Fiora?" and that would have started it just fine, but it wouldn't have done justice to the all-too-familiar topic.
"I admire you, you know."
"I- Vanea!" The shock was fresh, but the jump was weak. Who knew how much time there was left?
"It's what I've kept trying to do. To live on, for her. I like to think it's what she would have wanted."
"Vanea?" But Vanea just smiled, crossed her arms.
Every spare wire and brilliant orange wing was laid into place. Every flickering light and arcing stripe was painted with the care of centuries. Fiora would never dare to posit that this noble lady of the Machina was not made just right, just so, to bear up all her own most admirable responsibilities.
Vanea? Meyneth surely had loved her. And had she...had she loved Fiora?
If she had, then the context shifted, a little bit. Then maybe it wasn't just one-sided. Maybe that was the missing piece, the screw loose, the vice untightened.
By her lack of objection, Fiora was admitting that Vanea's assessment was correct. By her lack of confusion, Vanea was confirming that she had parsed as such.
"You know she's watching. She always has been."
Fiora gestured limply once more at the dormant core, then laid her other hand atop. Of course, Vanea was right.
"I know she remembers you. She always will."