They're not "missing", these hiragana characters don’t existed in modern Japanese language as it doesn't have these sounds so they did not need to be represented. You cannot write them in hiragana.

Some additional conventions exist to write foreign sounds in katakana but there are still limitations as both systems were originally designed for writing Japanese words. Until recently, these characters were only used to represent sounds that exist in Japanese that can be spoken by a Japanese reader. For example "wu" in chinese names is pronounced as "bu" in Japanese, "gim" in Korean names is pronounced as "kim" in Japanese, "va" in European names is pronounced as "ba" in Japanese. Furthermore "fu" and "hu" or "ra" or "la" are not distinguished in Japanese loanwords from other languages.

There are several exceptions for historical reasons. "wi" (ゐ) and "we" (ゑ) are used very rarely for names but are now pronounced the same as "i" (い) and "e" (え) respectively. There is no longer any need to write them differently as the sounds in modern Japanese are the same. Some brands still use it as their original historical name such as "Yebisu" (ゑびす/ヱビス) beer.

Another source of confusion is inconsistent romanisation. The "ye" sound for example, doesn’t exist in modern Japanese but ゑ was written as "we" or "ye" since romanisation was not standardised at the time. This is also why 円 is written as "Yen" in English, it was historically read as "wen" (ゑん) and is now read as "en" えん. So despite non-standard romanisation suggesting it, the sound "ye" doesn't have a hiragana character as it is not spoken in Japanese.

They're not "missing", these hiragana characters aren't needed as they don’t exist in modern Japanese language. The language doesn't have these sounds so they did not need to be represented. You cannot write them in hiragana.

Some additional conventions exist to write foreign sounds in katakana but there are still limitations as both systems were originally designed for writing Japanese words. Until recently, these characters were only used to represent sounds that exist in Japanese that can be spoken by a Japanese reader. For example "wu" in chinese names is pronounced as "bu" in Japanese, "gim" in Korean names is pronounced as "kim" in Japanese, "va" in European names is pronounced as "ba" in Japanese. Furthermore "fu" and "hu" or "ra" or "la" are not distinguished in Japanese loanwords from other languages.

There are several exceptions for historical reasons. "wi" (ゐ) and "we" (ゑ) are used very rarely for names but are now pronounced the same as "i" (い) and "e" (え) respectively. There is no longer any need to write them differently as the sounds in modern Japanese are the same. Some brands still use it as their original historical name such as "Yebisu" (ゑびす/ヱビス) beer.

Another source of confusion is inconsistent romanisation. The "ye" sound for example, doesn’t exist in modern Japanese but ゑ was written as "we" or "ye" since romanisation was not standardised at the time. This is also why 円 is written as "Yen" in English, it was historically read as "wen" (ゑん) and is now read as "en" えん. So despite non-standard romanisation suggesting it, the sound "ye" doesn't have a hiragana character as it is not spoken in Japanese.

They're not "missing", these hiragana characters never existed because Japanese language doesn't have these sounds so they did not need to be represented. You cannot write them in hiragana.

Some additional conventions exist to write foreign sounds in katakana but there are still limitations as both systems were originally designed for writing Japanese words. Until recently, these characters were only used to represent sounds that exist in Japanese that can be spoken by a Japanese reader. For example "wu" in chinese names is pronounced as "bu" in Japanese, "gim" in Korean names is pronounced as "kim" in Japanese, "va" in European names is pronounced as "ba" in Japanese. Furthermore "fu" and "hu" or "ra" or "la" are not distinguished in Japanese loanwords from other languages.

There are several exceptions for historical reasons. "wi" (ゐ) and "we" (ゑ) are used very rarely for names but are now pronounced the same as "i" (い) and "e" (え) respectively. There is no longer any need to write them differently as the sounds in modern Japanese are the same. Some brands still use it as their original historical name such as "Yebisu" (ゑびす/ヱビス) beer.

Another source of confusion is inconsistent romanisation. The "ye" sound for example, never existed in Japanese but ゑ was written as "we" or "ye" since romanisation was not standardised at the time. This is also why 円 is written as "Yen" in English, it was historically read as "wen" (ゑん) and is now read as "en" えん. So despite non-standard romanisation suggesting it, the sound "ye" doesn't have a hiragana character as it is not spoken in Japanese.

They're not "missing", these hiragana characters don’t existed in modern Japanese language as it doesn't have these sounds so they did not need to be represented. You cannot write them in hiragana.

Some additional conventions exist to write foreign sounds in katakana but there are still limitations as both systems were originally designed for writing Japanese words. Until recently, these characters were only used to represent sounds that exist in Japanese that can be spoken by a Japanese reader. For example "wu" in chinese names is pronounced as "bu" in Japanese, "gim" in Korean names is pronounced as "kim" in Japanese, "va" in European names is pronounced as "ba" in Japanese. Furthermore "fu" and "hu" or "ra" or "la" are not distinguished in Japanese loanwords from other languages.

There are several exceptions for historical reasons. "wi" (ゐ) and "we" (ゑ) are used very rarely for names but are now pronounced the same as "i" (い) and "e" (え) respectively. There is no longer any need to write them differently as the sounds in modern Japanese are the same. Some brands still use it as their original historical name such as "Yebisu" (ゑびす/ヱビス) beer.

Another source of confusion is inconsistent romanisation. The "ye" sound for example, doesn’t exist in modern Japanese but ゑ was written as "we" or "ye" since romanisation was not standardised at the time. This is also why 円 is written as "Yen" in English, it was historically read as "wen" (ゑん) and is now read as "en" えん. So despite non-standard romanisation suggesting it, the sound "ye" doesn't have a hiragana character as it is not spoken in Japanese.