also, again
亦 centers on addition or repetition: something that is also present or happens again. Its core idea is inclusion or recurrence.
existence (as we perceive it) is as mutable and incorporeal as is the morning dew or a flash of lightning
亦 is an ancient character originally depicting a person with marks under the arms to indicate 'armpits', later borrowed to mean 'also' or 'again'. The modern form is highly stylized and no longer pictorially clear.
The top 亠 looks like a lid, and the lower part resembles two marks being added. Picture a lid being placed again on a container, marking it as 'also' there.
For エキ, imagine a sticky note saying 'also' stuck on a bottle of eki (liquid): eki -> エキ, and the note adds the idea of 'also'.