Plaintext

The Plaintext stores a 12 or 15-word mnenomnic, much like hdwallet, but without any encryption.

Why?

It's a fairly common practice for developers to keep a mnemonic purely for testing purposes (local development, or interacting with testnets). In fact, the default wallet created when you first boot ethui is of this exact type, and you may recongize it if you ever used anvil:

test test test test test test test test test test test junk