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The Yoneda embedding is denoted in a few works by よ (the hiragana for the first syllable in Yoneda's name): https://ncatlab.org/nlab/show/Yoneda+embedding.

This is the only use of Japanese script in mainstream English-language mathematics that I'm aware of, although I'd love to be proven wrong.

A colleague of mine ran out of greek letters in his thesis. He added hebrew letters.

Aleph is a Hebrew letter I think? It's used for describing infinities: https://en.wikipedia.org/wiki/Aleph_number

Well, you'd skip that one just like you'd skip lower-case pi,