Magma came into existence as the Ki Rin, the Japanese Unicorn. She was never really born, but she was created from the spirits of the earth along with the others of her race; the Ivory Starlight Race. They were the embodiments of the purest souls and spirits of the universe. How the first two came to be is still unknown, and they took the forms of a man and a tiger who came to be called the Ancient One and White Blaze.

The other nine creatures of the race embodied the virtues of Trust, Wisdom, Justice, Life, Obedience, Serenity, Piety, Loyalty, and perfect Virtue and were born out of different regions of the earth. A mermaid was born from the seas and represented Trust, from the forests a Deer arose to be the living symbol of wisdom, the deserts formed a dragon which was the emblem of Justice, and a pegasus was born from the clouds and represented Life. In the cold arctic lands a wolf came to life to symbolize Obedience, while in the hot summer of the rain forests a spider wove a new web for Serenity, while a snake coiled forth from autumn for Piety, and a faerie twinkled in the twilight of a new spring for Loyalty. The Ki Rin, the Unicorn, was born from Mount Fuji, and symbolized perfect virtue, all the virtues in harmony.
 
However, as it's true that where there's a great light, there is also great darkness, the purity of the new creatures pushed away all the evil around them, and it formed itself into a demon. The demon, though powerful, came to fear the one known as The Ancient since his power was the strongest of the Ivory Starlight creatures' and could easily destroy him and the evil he represented. The demon soon learned, however, that he could posses humans and could approach the creatures of the Ivory Starlight Race without being harmed as if he were in his own true form. So, bent on getting enough power for himself so he could destroy his foe known as the "Ancient One", he possessed a human man and hunted down every creature, absorbing them and gaining their power for his own as he went, corrupting the virtues.

The last creature he intended to absorb was the Ki Rin in Japan, which he knew would give him enough strength to kill the Ancient and White Blaze (the two most powerful of the race). The Ancient sensed what was happening and, at the Ki in's request to have him protect her, turned the unicorn into a human. He separated her physical form and the soul virtue which made her a creature of the Ivory Starlight Race, and trapped the powers in a yoroi crystal
which she could call on to form a protective armor if the demon ever DID find her. This effectively made the first Armor, but she never had a chance to use it. 

The demon, however, still managed to outwit the Ancient and find Rin (as she called herself in her human form). When the Ancient  and White Blaze tried to come to her rescue and banish the demon to the Nether World, the demon had already had a hold on Rin and she was banished with him.

In the Nether World, the demon absorbed her and used her power to form his own evil armor. All of the other creatures had died when they became absorbed, but since Rin's power and physical form were separated, she did not die. She became linked to the powers of the others through the armor of Talpa which was created from her armor, and when  the Ancient  split the nine armors, not knowing Rin still existed in stasis, the Ancient effectively spilt her soul into nine pieces, the "Nine Rins".

When Talpa was finally destroyed, Rin awoke from the stasis, but couldn't survive without the nine halves of her soul.
The Ronins helped her restore her lost spirits and when they all joined again, she became Magma Angel. She swore to keep the legend of the armors alive so no one could ever forget. She took up her home in Mount Fuji and set up many caverns to depict everything that had happened, in the hopes that one day someone would find them and know everything that had occurred.

The Ronins have no memory of what happened with Rin/Magma Angel, and Magma has no real wish for them to find out who she really is, or she thinks it will ruin the new life she made when she was adopted by the Date family and became Sage's cousin and met them all over again.