I suggest you read this article https://en.wikipedia.org/wiki/Historical_kana_orthography https://en.wikipedia.org/wiki/Historical_kana_orthography It explains

I suggest you read this article https://en.wikipedia.org/wiki/Historical_kana_orthography https://en.wikipedia.org/wiki/Historical_kana_orthography It explains the shifts in pronunciantion in Japanese, making old Japanaese kana ortography differ from actual pronunciations, as in English orthography today. The remains of the old system can still be seen in the particles: A particular shift is medial and final h/f->w and w->∅(except) for 'wa'. This meant that non initial は was pronounced 'wa'. For example, かわ 'kawa' (river) was written as かは before the reform. The かは reflected and even older pronounciation of 'kapa' for the word There's a shift p->f->h on the は 'row' explaining why ふ is 'fu', and why ば is 'ba', as 'b' is the voiced equivalento of 'p' and not 'h') There was the same thing for へ, かえる 'kaeru' (frog) was written かへる historically, reflecting these changes. From wiktionary: ⟨kaperu⟩ → /kaperu/ → /kaferu/ → /kaweru/ → /kajeru/ → /kaeru/ (notice we -> ye -> e shift of ゑ here too, this one recent enough to influence the word 'Yen') Source: https://en.wiktionary.org/wiki/%E8%9B%99#Japanese https://en.wiktionary.org/wiki/%E8%9B%99#Japanese So the particles は and へ, being pronounced 'wa' and 'e' instead of the expected 'ha' and 'he' are just remainers of this old orthography conventions, and give hints that the old form where probably pronounced 'pa' and 'pe' over a thousand years ago! Facinating stuff indeed :)