The "default actor" process is a bit hard to parse ATM, and this is its linchpin.

* Actors (called sprites) have Costumes (currently called skins, maybe I'll change them to costumes to disambiguate)
* For the K1 levels, some default Actor <=> Costume pairs are initialized (and players can never change the costume of a given actor). These dropdowns only apply to those default pairs
* Levels specify these with "spriteStartingImage" and "spriteFinishIndex" parameters in levels.js, which get looped around in initialization

So re: your question, there could eventually be non-starting costumes (one that's a costume option but never pre-populated for a level)

Maybe next time we add some new characters in could be a good time to switch over to simply including a list of default costumes in each level definition.