Sometime in the early 20th century, usage of the now-historical kana ゐ and ゑ (and their katakana equivalents) dropped off, being replaced with い and え in modern Japanese. What exactly happened here ...
In the last century, the ゐ and ゑ characters were eliminated from common use. But it seems like there used to also be a "wu" character that has since been lost. Given that it's a lot harder to find ...