Context:

Edith was planned to be a playable character but got scrapped, because her game mechanic was too difficult and there wasn't enough time to rework her, so she instead got implemented as a rare easter egg.

The easter egg can be found by using the reverse card "VII - The Chariot?" which usually turns you into a regular statue for a few seconds, but on rare occasions turns you into Edith instead (cosmetic only).

To unlock that card, simply beat challenge number 42 "Hot Potato" which requires you to unlock Tainted Forgotten first.