A Soul Star is the physical manifestation of the essential core of a character. Each Soul Star has two functions. Beings who do not have a Soul Star are called Illborn.
Soul Stars accumulate Soul Charge, which is generated by strong emotion from the owner of the Star, and use it to perform their functions. Soul Charge can be transferred between Stars, which was used to turn Illborn Riltia back to normal and is theorycrafted to have been the way to save Yaos.
Second Shift turns a Star into its weapon form, which usually has an additional effect.
Normal Illborn have had their Soul Star separated into its two components, the binding and the Essence. Willborn have full Soul Stars, but without Charge.