Kupira
Kupira is a young world, only as old as the Hivan War. That's not when it was discovered; that's when (according to the inhabitants) it was made.
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Razing of the Upperlands
The populace of Kupira are descendents of the peoples of the Upperlands Forest in Mana. The Upperlands was where the Hivans landed their invasion colony in Mana, and so the forest endured horrific destruction both as a result of Hivan colonization and the Grand Army's campaign against the same.
At the end of the war, the people who had evacuated the Upperlands returned only to find that their home was not the same -- and likely would never be. Many chose not to return at all, instead becoming citizens of Pandora, Kakkara or Tasnica.
Prayer for Wind
Those that did return to the Upperlands did their best to return the forests to health. While new growth did occur during this period, the young forests paled in comparison to the towering, millennia-old trees that once dominated the forestscape. Animals, too, were slow to return to the region, making the carefully balanced livelihood of Upperlands forresters a thing of the past.
In the end, these Upperlanders decided that they would have to leave, too. But rather than settle for attempting to emigrate to another country, this second group of emigrants chose to seek divine assistance to recreate what they had lost.
Kuuter, a Moogle shaman dedicated to the Mana Elemental Sylphid, entered an extended period of austerity, meditation and prayer before, according to legend, being guided by the wind to a clearing in the new forest growth, where there was a previously undiscovered portal. Beyond this portal was the world of Kupira, which is where Kuuter lead the survivors of the Upperlands and guided them in reestablishing their old ways.
Debate over Origins and Classification
No one is completely certain whether or not the Kupirans' tale of their world's origins -- that Sylphid crafted the world and its star system in response to Kuuter's prayers -- is true. While the portal into Kupira from the Upperlands does, in fact, exist, a handful of additional portals into Kupira have also been found in the Mana, Carrion and Kuvalla dimensions.
There is also the larger question of whether or not such a feat is even possible: greater powers than a mere Mana Elemental have not previously displayed such a potent ability as the power to just create a whole dimension out of thin air. And yet, the Kupirans firmly (and in the face of all criticism) attribute their creation myth to Sylphid.
Kupopolis Neo
In the Neo timeline, Kupira voluntarily became a province of the Thebean Empire. The Thebeans called this world Sylva, perhaps as a likely reference to the Kupirans' creation myth.