Where are Wi, Wu, We?
Curious as to why there are no letters for those sounds, given that there is for Wa and Wo(even if its pronunciation has changed). Also it's interesting that Wu is a very common sound in Chinese, and the two countries don't have a long distance between them.
Interestingly enough, wi and we used to be sounds that existed in Japanese and had their own kana (ゐ/ヰ for wi and ゑ/ヱ for we) but have become obsolete.
The sounds shifted to い/イ and え/エ at some point between the Kamakura and Taishō periods (which is to say, at some point between 1333 and 1912, which is a pretty long time). The kana were deemed obsolete 1946, but still made it into Unicode for some reason (which is the only reason I'm able to type them).
Actually, as some restaurants and companies like to have a more 'traditional' name, they will use these old kana. Ebisu beer, for example. While it is not necessary to have the same familiarity with them as with the kana commonly used today, it would be beneficial to at least be able to recognize them (assuming you are aiming for a greater level of fluency).
Wi ウィ Wu ウ We ウェ Wo ウォ or ヲ These are all pretty limited in that they are used fairly exclusively for foreign word representation you will likely not find them in any earlier study material.
Chinese and Japanese are two languages with completely different roots. Just because they are relatively close geographically doesn't mean that they will use the same sounds. It doesn't mean anything. Korea is much closer and doesn't have the 'wu' sound either...
Two of them existed when the Shingon Buddhist priiest Kukai invented hiragana, and are in the Iroha poem that used to be attributed to him, with each kana appearing once.
"Note that archaic hiragana uses ゐ and ゑ, which are now only used in proper names and certain Okinawan orthographies."
There was never a kana for wu, which is impossible in Japanese. It would just be uu.