Japanese is phonetic, but there are many homophones in Japanese, so writing under a pseudonym will cause considerable semantic confusion. In order to avoid this, Chinese characters are introduced to express meaning.
Hiragana corresponds to Katakana one by one, and each Hiragana has a corresponding Katakana, which is equivalent to the relationship between uppercase and lowercase letters in English. Now Japanese generally uses hiragana and katakana to represent loanwords.