Funny story - I found an incomplete version of this chapter in an old file from 2015. So I polished it up and added a lot more to it, and fixed things so that it aligns a little more closely with both canon and this story's version of events.
I hope you enjoy it.
Many Years Ago…
Many years, many centuries had Mard Geer wandered the world, traveling far and wide in search of the power that would allow him to fulfill the purpose for which he'd been created. With E.N.D. in hand now though, he sought out the rest of the scattered demons. His creator had called upon him, and the Underworld King would respond. If it was time to reorganize Tartaros and end his travels, so be it.
The designated convergence point was, however, regrettably distant from where Mard had been previously. And as he found it rather difficult to stomach the presence of a large group of humans - or indeed, any humans at all - this meant that he had to walk there. Though Mard Geer had been walking from place-to-place for centuries, so really it wasn't that much of an imposition overall. His body was created to be much more durable than a humans, and he only needed to draw sustenance when he was injured, from the ambient ethernano particles in the air. Several days of nonstop travel and he would arrive, likely ahead of the rest.
His purpose was clear - resurrect Tartaros, grow its ranks, and perform Zeref's will. It was clear, and concise with no room for misinterpretations.
Mard eventually found himself traversing an unfamiliar mountain ridge. When had a mountain appeared here? Mard didn't recall a mountain being in this location previously - then again, it had been... quite some time since his last visitation. But surely not that long?
He did not wish to entertain the idea that he was, perhaps, a little lost. It wasn't as if anything that dwelt there could kill him, but it was rather unpleasant to not have his bearings. However, the energy signature he was tracking was easy enough to follow – the escaped demon did little to hide her presence, believing that she had already attained enough distance to free herself from Tartaros and her fate. As soon as he was finished dealing with her, then he could descend the mountain and continue on his way to where the Plutogrim slept. The scenery, of nature that had yet to be completely soiled by the humans' filthy hands, was pleasing enough to look upon as well, and he was in no great hurry.
Finally, he spotted her hovering over a ridge far to the west of his current position.
Hair of rippling obsidian night flowed upward with the swirling magic that surrounded her. Wings of sable unfurled from her bare back, a thick tail of jet thrashing the ground beside her golden, clawed feet. The cruel talons of her hands flexed as stygian energy coalesced into spheres of dark power.
Raising her finned arms to the sky, his quarry set free the curse she had gathered, the sky cracking and the fabric of the world warping as the rapidly spreading void twisted and wrung it.
Mard watched in interest, feeling the tides of energy ripple across his skin. It couldn't do any damage to him from this distance, and it was always spectacular to see. Approaching her on foot would be a fruitless exercise, with wings at her disposal she could flee much more quickly than he could pursue, and he did not wish his pursuit chase to be extended unduly. So he would wait until the precise moment he needed to strike with his thorns, when the distance was much less, and enjoy the show she was putting on for now.
When the pulsating waves of darkness receded, the demon swooped downward, vanishing from Mard Geer's sight.
Several hours it took him, to reach the spot where she'd disappeared. Plumes of smoke rose above the ledge, and Mard Geer smiled at the remains of the valley below. Where a mountain village had once nestled, now lay only smoking ruins.
It was good that Satan was having her fun, Mard Geer concluded. It would be put to an end with her death, soon enough, so it was her final opportunity to do so.
And it only left a better trail for him to follow.
The Present
Mirajane wondered, later, if she'd done the right thing in telling her family and the world at large about the man that had helped her all those years ago. It was a very private memory, one she'd held close to her chest and her tongue still in order to protect, and it still didn't feel quite right to share it with others. She'd had so little back in those days, and had felt so little kindness from other people. Meeting him, and having been taught by him patiently (if brusquely), had meant the world to her and she'd clung to that image she had of him in her mind just as fiercely as she did to her siblings. It was immeasurably important, what he'd done. And the memory of it belonged to her.
Despite her confliction over the rightness of the action, she did understand why she had done it. In announcing her story to the world, it was possible that somewhere, someday, he might see it. And remember the terrified girl with white hair that he'd helped. That he would know that she turned out okay, in the end.
Not that that had worked out for her, in the end. She'd vanished for seven years after that revealing interview, in a blast of dragon's breath.
He might also be dead, himself. After all, he had stated directly to her face that he wasn't exactly young. And that had been almost seventeen years ago, now.
In the end, she'd put a vulnerable side of her on display for nothing in return. And what had been revealed had made her siblings sad, something she'd never wanted to do. They'd had enough sorrow in their lives as it already stood.
Besides which, when she reflected on it, she hadn't been entirely truthful in that interview in the first place. There were things she'd held back from both Sorcerer Weekly and her siblings about the man.
In particular, it was in what her feelings for the man had been named.
Calling what she'd felt for the man a "crush" was putting it too simplistically. In hindsight, there really wasn't a label that fit him, what he'd meant. Which was why she'd gone along with the description. She had nothing better to offer, herself. He'd given a little bit of light to the darkest part of her life. Shown her a way to coexist with the monster that dwelt in her heart. There was no word for that.
Except, maybe, "teacher."