When thinking of a way to help Humanity the gods gave the people of Gekai their children, Spirits.
Of all the spirits the First, Second, and Third Gen were the most powerful.
First generation's like Nox are only dwarfed in power by the gods themselves. They took to much after them and were not very human compatible. In most cases the contractors all die from exhaustion of the Spirits using their magics, other's die along with their contractors as per their contractors dictating the Spirit give their existence to them. Such contracts were very, very common.
Of course the first generation knew of their flaws from their fellow generation discovering it. From that knowledge lived by making shallow contracts which allowed them to help humanity for much longer. But when the gods gave out their Falna the true might of the first generation reared it's face. When using the Falna of their contractor they can amplify their existing magic and bring it up to the realm of gods. After this was learned the Spirit Hunt began leading to the first generation to go extinct and other generation spirits to be killed from their respected god's relationship with one another.
Second Generation Spirits like Aria (Ais's mom) were the most Humanity compatible, a little to much. When their contractor uses their spirits magic they are able to fight a Spirit of Nox's caliber, granted you will need at least 10 or maybe 15 to try and kill her (When Nox is powered by a Falna it would take at least 50 to kill her alone.).
Much like with the first generation the second also had their devastating defects. When they make a contract with one of the opposite sex they can have children with them alone. Children of the Spirits were named Nephilim. Their has only been one who lived to adulthood, the rest were slaughtered because of that adult's actions and strength.
Third Generation, these Spirits were the perfected recipe the gods finally mastered, they had a good balance of strength and human compatible. In this generation you have a lot of Spirits who turn into weapons and objects.
They are the basis in which Spirits were created from then on.
Fourth Generation. They are merely normal Spirits who were created for quantity over quality. Their nothing special about them, aside from the Heroes they contracted with.
There is classification on a Spirit's strength.
- Grand Spirits. They are entirely comprised of First Generation Spirits.
- Spirit Kings. They are comprised of the Generation 1.5 and some from the Third Gen.
- Great Spirits. They are made of mostly of the Third Gen.
- Spirits. They are entirely comprised off Spirits from the Fourth Gen.
- Unique Spirits. These Spirits vary Spirit from Spirit in this classification. These Spirits are majorly made from the in-between of Generations so they are comprised of Gen 1.5 and 2.5's.