First referenced:
The Portal in the Forest (Part 4)
The Regret Demon is a mostly unknown entity that presents trade options that invariably lead to remorse and regret. The Regret Demon was first encountered by the unnamed brunette woman after it hijacked the primary forest portal to discuss a truce.
Traits:
Apparently a large organism, capable of redirecting interdimensional portals and generating tangible illusions or matter. Comprised of neural tissue in a similar fashion to the Dreamer On High entity, the Regret Demon has at least one (and likely more) enslaved humans under direct mental control.
Displays hostility and fear towards the Soul Reader (Book), likely due to its ability to read information forcibly from souls without the need of providing payment.