Back in 2009, I wrote one of my earliest essays on the nature of functionality in symbolic discourse, DON'T FEAR THE FURNITURE (and an addendum, Part 2). These distinctions about "simple and complex variables," an idea developed from one of Frye's definitions, eventually became subsumed by the language-terms introduced by Philip Wheelwright's THE BURNING FOUNTAIN, last cited here. While I don't dismiss the algebraic metaphors of Frye. Wheelwright's physics-influenced metaphor has proven more useful in trying to map out just what literary process separates the simple from the complex.
As a roundabout way of refining this question through example, let me say that while I still view all of the "mythcomics" I've cited as worthy of being called "symbolically complex," I've observed that sometimes even characters who possess that potential, that amplitude, have been treated like furniture: i.e., as merely functional.
"Secret of the Sinister Sorcerers," analyzed here, shows this tendency.
All of the heroic characters on display in this page have sustained, at one time or another, strong symbolic discourses in their own features. One might argue, in line with Wheelwright, that at that time Aquaman and the Martian Manhunter weren't "eminent instances" on the same level of the other heroes, perhaps because the two heroes had spent so much of their respective careers as short back-up strips. But in "Sorcerers," they're all on the same plane, for author Fox isn't mining any of the heroes' myths to any great extent. In terms of symbolic complexity, the three villains have the greatest amplitude of associations, while the heroes simply run through their functional paces: Green Lantern's ring can't battle a yellow manticore directly, so he has to defeat the creature indirectly, etc.
More often than not, though, mythcomics tend to imbue super-functional characteristics to both protagonist and antagonist. This is certainly the case in GREEN LANTERN #40, where the titular hero, the mentors he represents, and the villain Krona get a great deal of myth-attention-- though one might argue that the Green Lantern of Earth-II isn't much more interesting than your average piece of used furniture.
This is a general tendency evinced by many "sidekick" figures. Lightray of THE NEW GODS is not without some symbolic associations of his own, but he's primarily important as the friend of the book's hero Orion. Because Orion likes Lightray, so does the reader, and thus does the reader become more invested in the hero's struggle on behalf of New Genesis. Lightray isn't as "eminent" an "instance" as Orion, but he's eminent enough for Kirby's overall purpose.
As I observed back in the FURNITURE essays, story-elements that are merely functional-- like the "stairhead" allows "stately, plump Buck Mulligan" to enter a room in ULYSSES's first line-- are both inevitable and desirable. The same principle applies to characters whose complexity varies from story to story, according to the needs of the author. It's especially interesting, at least for a project like mine, when the mythic complexity inheres more in the opponents of the hero than in the heroes who are the putative stars of the story, as in "Sinister Sorcerers."