also, again
亦 centers on addition or repetition: something that is also present or happens again. Its core idea is inclusion or recurrence.
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'.
existence (as we perceive it) is as mutable and incorporeal as is the morning dew or a flash of lightning