Dewey is a spirit of knowledge residing in The Garden, a library that Team Mosaic uses as their base. They have taken up residence in a blank worn green book nestled in one of the main bookshelves. In order to communicate with Dewey, one must open the book and allow Dewey to temporarily possess their body, in which they will be able to talk to the outside world.

Dewey has knowledge of any written piece of work, both from Cartesia and from Earth, and can freely replicate them for others to use. They're one of the main reasons Cartesia has such knowledge of Earth. Dewey has become obsolete in recent years, which is why they've been relegated to living inside an abandoned-until-now library.

Dewey is very flamboyant and grandiose in their speech and mannerisms, but they value honesty over pretention. Every time somebody new joins Team Mosaic, they ask them what their favorite book is as a test. If they answer honestly, no matter what it is, they pass. To Dewey, all written works are valuable and deserve to be protected.

Dewey can manifest the contents of any book, from Cartesia or Earth.

The origins of Dewey are unknown, but there are theories that they manifested after the burning of the Library of Alexandria, as a way to prevent such a tragic loss of knowledge from ever happening again.