Yui makes more sense to me than Ewey in written form, since Yui is a Japanese name.   Ewey would be more along the lines of how to properly pronounce Yui.   Also, it appears that the term Yui is a girls name and appears to mean Elegant Cloth, as well as alone, which would be rather relevant to this character's abilities.
https://en.wikipedia.org/wiki/Yui_(name)    also other google searches.