In Reveal Novum, Book's definitions of Coiny and Firey say:

Coiny - likes to slap Firey. But what I don't get is how you slap fire. Won't your hand just go right through? Whatever.
Firey - Firey is fiery. Note that they are spelled differently.
In Get Digging, Golf Ball discusses the means of getting yoyleberries with Firey and Book.

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In Zeeky Boogy Doog, Firey tells Book that she must not be very well-read. Book takes offense to this and injects Firey with freeze juice.

In Welcome Back, Book mentions while walking to the HPRC with Pencil that "we've gotten Firey punished, and all."