Katakana — ヂ

Unlike hiragana ぢ, which still survives in a handful of native compound words, katakana ヂ is virtually unused in modern loanwords — ジ covers the "ji" sound universally for foreign words. ヂ mostly appears only when directly transcribing the hiragana ぢ into katakana for stylistic or technical reasons (e.g. dictionaries, IME input charts).