Being a perpetual re-reader of the books, I made a list of playable characters present in each book as a sort roster to choose from.
I listed them separated into "Significant Ally" and "Present", as some are definitely less helpful than others, even sometimes more of a hindrance than a help (such as Murphy prior to Summer Knight, really).
The distinction between Ally and Present is of course pretty much arbitrary, and debatable in several cases, but here goes:
(Note: I refer to the character "Billy & Georgia" as "Alphas" below)
Storm FrontSignificant Ally: None, actually!
Otherwise Present:
Murphy, SusanFull MoonSignificant Ally:
Susan (almost put her as Present here too)
Otherwise Present:
Murphy, Alphas (the Alphas don't actually accomplish terribly much in this book, Murphy is an obstacle really)
Grave Peril*
Significant Ally:
MichaelOtherwise Present:
Susan, Thomas, Murphy (Murphy's out for a lot of the book, Susan & Thomas are as much complicating as helping)
Summer KnightSignificant Ally:
Murphy, AlphasOtherwise Present: In a "know your lore or you'd never know" we actually have both
Ramirez and
Luccio on screen for about a line each, unnamed.
Death MasksSignificant Ally:
Michael, Sanya, Susan ("upgraded")
Otherwise Present:
Murphy (might well fit above),
Butters, Molly (technically)
Blood RitesSignificant Ally: Murphy, Thomas
Otherwise Present: Mouse (Origin Story!
)
Dead Beat* (So many, several are on the border between Ally and Present)
Significant Ally:
Butters, Ramirez, MouseOtherwise Present:
Thomas, Luccio, AlphasProven Guilty*
Significant Ally:
Murphy, Thomas, MouseOtherwise Present:
MollyWhite Night*
Significant Ally:
Molly, Thomas, Mouse, RamirezOtherwise Present:
Murphy, ButtersSmall Favor (as with Dead Beat, distinction is fuzzier than usual)
Significant Ally:
Michael, Sanya, Thomas, MurphyOtherwise Present:
Mouse, Luccio, MollyTurn CoatSignificant Ally:
Thomas, Molly, MouseOtherwise Present:
Murphy, Luccio, Butters, Alphas*A Variant Card for Harry is fitting.
And to relist the sources of the characters:
Butters and
Thomas is in
Expansion 1Sanya and
Molly are in
Expansion 2Luccio and
Ramirez are in
Expansion 3Mouse is in the Evil Hat exclusive "
Mouse & Variants" mini-pack