The rats are a race of highly-intelligent rodents, seemingly possessed of a collective mind. They watch over the world of T.O.E. in secret, often serving as a source of food and materials for the other races. The rats don't seem to mind being viewed as a commodity.
The rats appear to have powers above and beyond those of the average creature. When grouped together they have access to magical abilities, such as shape shifting and summoning, and though they can't communicate verbally with other races they can use spirits to relay messages. The rats of Castle __________ have employed Philip the Ghost for this purpose, probably against his will.
The rats are among the few creatures that seem to know they are characters in a video game. They may also serve a role above and beyond mere NPCs - Iko refers to Traveller's rat as a 'regulator' - though the nature of that role is currently a mystery.