The specifics of being an Elite Guardian haven’t been thoroughly explained in the comic until now. Basically, Elites are able to take the form of other people (and become fully visible upon doing so), but are limited in which forms they’re allowed to take. The basic rule of it is that it has to be someone who — at one time or another — actually existed.
This is basically just a measure to keep Elites from turning into any random creature they can come up with.
Thanks to both MajorMattMason and RandoChris for volunteering their characters to appear circa 1930!