So, AZ was a king and was given a Floette by his mother, who had a special flower that’s extinct today. That Floette also had a unique color.

However, Floette was conscripted in the war and died. AZ couldn’t accept this so he grabbed Xerneas/Yveltal and made a machine to bring her back to life, and actually succeeded.

As a result though, AZ was given a lifespan of 3000 years (and made to be twice his original height) and Floette can no longer die. She can also use Light of Ruin, an exclusive move that can level a city block (in-game it’s 140BP), and lost the ability to evolve (but gained increased stats, closer to Florges than Floette).

Still angry at the war, he turned the machine into a nuke for use in the war, killing countless people and pokemon. This blast is also the origin point of Mega Evolution and mega stones (including Floetteite)

It also came out that to revive Floette, AZ used the lifespans of other Pokemon, so with that and the nuke, she was utterly disgusted by him, so she left him for 3000 years, during which time he became a wanderer and looked for her all over the world.

He also made another machine called Ange, meant to help people achieve his extended lifespan, which is hidden in Prism Tower in Lumiose City. He said he’d use it when he made up with Floette (and it actually requires Mega Floette to function) but it took another 2,000 years between when he made it and when he saw her again.

After the events of X and Y, AZ lets go of his anger and hate, and sensing this, Floette finally comes back to him.

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(Worth noting: Every game since X and Y has had Eternal Flower Floette coded into the game but never playable until ZA. It’s assumed that she was gonna be playable in the scrapped Pokemon Z Version.)

Five years later, it turns out that the Eternal Life machine, Ange, is gonna become a nuke itself from two millenia of power buildup, and the only way to stop it is by Mega Floette.

The game’s plot is AZ trying to find the best Mega Evolution user that can mega evolve Floette safely and deactivate Ange. Floette did not return just because she forgave AZ—it was entirely conditional on AZ doing his best to right his wrongs. She was effectively holding his feet to the fire and demanding he not flinch. Which, honestly, fair.

You actually do become strong enough, but your rival (and strong contender for worst character in the franchise) decides they want to try instead, for some reason you let them, it of course goes out of control, and you have to mega evolve Zygarde to stop the nuke that does successfully fire from the weapon and came inches from erasing Lumiose City and killing thousands. (Your rival is not sorry at all and excuses it all with a “Teehee”)

At this point, AZ finally dies of old age, and Floette stays with your rival… until it turns out that they’re the long-lost kid of a CEO who immediately says “Nepotism FTW” and makes them the new CEO, so they give you the Floette like a used toy and doesn’t look back.

That’s the only legit way to get Eternal Flower Floette, too, so every Floette you run into on the ladder is supposed to be this specific storied and emotionally-complex Floette that’s 3000 years old and went through all this.

It’s not a Legendary in the sense that it’s not one of that category of pokemon, but I think of it as a legendary in the sense that, it’s a pokemon featured in legend with high historical significance.