Characteristic: Hates to lose
Nimue tends to be more of a pessimist; primarily assuming the worst of people and how they'd be more selfish than helpful. It also doesn't help that she's seen many of them live and die, while she managed to keep herself alive for the past 1,013 years, and therefore, few patterns of their behavior escape her notice. As a result, it is very, very difficult to befriend her, or even gain her trust, as how can she expect your help when she'd be alive long after you'd die? Having seen much of life, especially the massacre of the Fairy people in her earliest years, and isolating herself with mainly ghosts most of the time, she's prone to not having much tact and just saying whatever's on her mind to whoever she's speaking to, whether it be good or bad. Recent events may have given her new hope, but thousand-year-old habits die hard.
As of now, she's rather cautious of the people that now inhabit Aether, and will only ever show herself to the people who have collected at least three Heart Scales and can use one of her Mega Stones, or if they can't, then six Heart Scales for a new move. Also, don't call her a witch. She dislikes the negative connotation such a word has (even if, as a Fire-type, it would be rather difficult to "burn" someone of her caliber), and would be much rather called an innovator, since that's pretty much what she does.
Nimue's earliest memories consisted of the old kingdom of Aether 1,000 years ago, back when the Fairy ancestors of the Pelaruna tribe used to rule. During those precious few years, she really enjoyed their magical power and wanted to be just like them. Unfortunately, tragedy struck a year just before her 13th birthday, when she was promised that she would be taught their magic. The fiery, bloody massacre of King Jackson and the Fairies at the hands of the Dragon people happened and the Celestial Stone was drained, crushing her hopes, dreams, and nearly her life.
She couldn't understand why this happened. For a long time, she tried to get revenge by defeating the members of the Dragon people, but she would always lose. Disheartened, she then tried to track down the few remaining survivors, Anzic the Clefable and Bazel the Jigglypuff in particular, but by the time she did, they had already decided that they could not possibly revive the kingdom, nor would they ever tell their descendants what actually happened so they would not feel any sort of longing for what they lost. Though she did her best to try and change their minds, they remained unshakable, and she eventually abandoned them and their Pelaruna tribe, disillusioned by the Fairy kingdom and what it promised her.
Instead of watching the now Nizhoni tribe live out their simpler lives far away, Nimue decided to return to the ruins of Aether, which was now a dead ghost town. She could talk to Dullahan and Willow at times, but for the most part, she shared the ghost people's sentiments about being unfairly murdered just like that, and wanted to help them eventually get their revenge and/or bring the kingdom back to what it once was.
But with the latter option not being available, she decided to focus on the former instead. She became obsessed with making herself more and more powerful over the years, resorting to learning all the ways of magic that the Fairy kingdom had, and whenever someone she suspected of being related to the Dragon people came around, she would test her new skills on them to try and kill them, earning her a rather fearsome reputation, as well as the rumors around the old kingdom of Aether being haunted by ghosts and witches, and therefore too dangerous to live in. Even after she evolved all the way into a Delphox and trained and made herself more and more powerful, she was distraught by how, despite all her power, she couldn't possibly go up against an entire country filled with dragons.
Regardless, Nimue's determination never faltered, and she even learned magic to help keep her young and powerful for the next thousand years. Though she got a bad reputation for dealing with dark magics that no normal person dared to touch, she didn't care. So long as it would help her in her goal, it was the only thing that mattered. Among these thousands of years, she traveled to various countries to learn more magic spells, and even came across the existence of Mega Stones.
Such a discovery consumed her, especially since the only major spells she seemed to be able to master effectively were those that granted abilities and moves that a Pokemon could never learn normally, as well as turning them into an alternate color. She wanted to learn all their secrets; what they were for, how they were used, and if possible, if there was one for her so that she could gain their power too. Still, though, she also realized that if she even wanted a glimmer of hope to be able to stand up to the Dragon people, she would need more allies...an army, ideally. But how could she possibly convince them to be on the same side as her?
An interruption to her work came when she noticed King Alaric's people stumble onto the ruins of the kingdom, and she hid in disgust, wondering what they were doing here, defiling the land of the fairies. ...At least until she heard Pythia's prophecy that these people might be the prophecised army she was looking for, and furthermore, a single descendant of Anzic and Bazel's had arrived. This small shred of hope turned into a full blown one when the entire Pelaruna tribe had come back and had restored the Fairy typing to both themselves and the world just in time, and she learned that the reason the type was gone the whole time was because King Jackson was forced to use all of the Celestial Stone's power to stop the army from completely destroying all the survivors.
Now, as Nimue sees how many people have come to Aether and repopulated it, as well as the Pelaruna tribe happy to rediscover their heritage, embracing it, and guarding the Celestial Stone, she feels way more content and at peace than she's ever been in centuries. She realized that maybe she could get these people's help in case the Dragon people ever attack again, and has successfully created a few Mega Stones for certain species. However, she is well aware of how great power can be misused in the wrong hands, and will only ever give such stones to people who have proven themselves to have a good heart by earning Heart Scales from good deeds. For those few truly dedicated, good people, though Nimue will certainly still be a bit cautious around them and act a bit suspicious, deep down, she's happy that her work all this time seems to finally be paying off, and that she got to see King Jackson's descendants finally return to the place where they belong.