When a reyvateil completes a level of her cosmosphere, she generally generates a set of clothes that represents the part of herself that was dominant on that level of her cosmosphere. For instance, a girl who feels hemmed in by tradition while still desiring to honor it might have a elegant kimono, or one who enjoys control might eventually generate a Dom outfit with spikes and such.

In addition to being marks of advancement in magical skill, they also tend to have the qualities of armor as well-they're made entirely of magic so their properties aren't limited by physical laws. As a general rule, if you see a fighting Reyvateil in what looks like a wedding dress, she's at the apex of her skill and power.