Illborn are beings that have had their souls removed, and most undergo the process of Illbirth; seperating the fundamental parts of the Soul Star itself, the Binding and Essence. They make up the majority of those living in the Warrens.
Willborn are beings that had souls, but, according to many theories, ran out of Soul Charge without having gone through the process of Illbirth first, having a complete Soul Star during their Form change. Although they are a type of Illborn, they appear to be multitudes stronger, and so far have only been defeated in a Spirit Duel, and OVERSOUL. Currently, only two Willborn are known. The first is often called Rillborn, and occurred when Riltia fell into the acid in Outer Underside, and had her Soul Star tampered with. The second is Yaos's Soul Core, which was created when the Insolvent Thaumaturge reached the final number in it's menu, depleting Yaos of his Soul Charge. Rillborn had it's Soul Charge restored by Ciro when (s)he entered Oversoul, reversing the effects of becoming an (w)Illborn. Yaos's Soul Core was entered, subsequently defeated and purified in a Soul Duel, seemingly destroying the star in the process.