emphatic final, interrogative
呢 centers on marking a sentence as emphatic or turning it into a question, functioning as a final particle that adds force or inquiry.
呢 combines 口 (mouth) with 尼, which likely contributes the sound. The mouth radical fits its use as a sentence-final particle expressing emphasis or interrogation.
The mouth 口 beside 尼 suggests a spoken particle that adds emphasis or turns a statement into a question, like a vocal tag at the end of a sentence.
For ジ, imagine a speaker adding an emphatic 'Gee!' at the end of a sentence: Gee -> ジ, and the mouth 口 emphasizes the point.