I've had a realization about Spirit Evolution, in terms of what is it. Basically, it's an overlay. According to the definition of overlay that I found regarding data, it is one of two or more pieces of data that can be loaded to a predetermined memory region on demand. Additionally: it can be stored like any other piece of data, it can be changed with another overlay at any time and only one overlay can occupy that space at a time. Sounds familiar, right? - Drakohahn
When Takuya and Koji gained the ability for Unity Spirit Evolution, it always bugged me that the Ice Spirit was a key part for the Fire Spirit. However it just hit me not that long ago; the Ice Spirit for the Fire Spirit and the Darkness Spirit for the Light Spirit were for control. All the other spirits were used to up the power of the Light and Fire Spirits, but they needed a counter productive spirit in order to have that power be controllable.