How to make a minor character that symbolizes an element in the story.
This one's tricky. Nebulous. I'm grasping at straws...
Maybe I'm just splitting hairs with this last category here, but I want to differentiate between theme and story element. If a theme is a mini-moral, a minor building block of the "so what," then a story element is an archetype of one of the story ideas.
The easiest example is a "Good vs. Evil" story. The good guy represents Good; the bad guy represents Evil. The good guy isn't a theme; neither is a bad guy. Themes hang off these characters, but the characters are bigger than that, more fundamental than that.
Not every story needs these types of characters. If handled well, they make a story more mythical, more fable-like (which may not be what you're looking for). If handled poorly, they make a story into a joke, a fake parable, the kind of thing you roll your eyes at.
The difference, as far as I can tell, is how individualized you make these characters. Are they both human and archetypes? Another good example of an elemental character--again, not a minor character--is Elric. He's both human (full of conflicting emotions and desires) and elemental (Servant of Chaos). Tying Elric's humanity to his chaos, making his emotions and desires the driving force behind his destructiveness, is what makes him so great. Michael Moorcock doesn't just slap an archetype on the page--making him a stereotype--but makes Elric's character a necessity for carrying out his elemental nature.
Spiderman. Same thing.
Elemental characters aren't stereotypes because the author narrows down her Big Idea. Darth Vader isn't just Evil. He's Evil, but he's controlled by other people--suddenly, he's not just a stereotype. Darth Vader is loyal. He's dependable. In the end, we find out it's love that's fueled driven him to this depth.
And, speaking of Darth Vader, that finally brings us to minor elemental characters.
Emperor Palpatine is a minor elemental character. The general idea for building one is the same as for a major elemental character--but the minor elemental characters stand in relation to the main character, just as their Big Ideas stand in relation to the main Big Ideas. Darth Vader is Evil. Emperor Palpatine is worse--but he's still an individual. His greed is destroying him, but man, that greed has led him to some pretty powerful places.
His purpose is to say, "Darth Vader? At least he's not Emperor Palpatine." In other words, the very idea that there is an Evil is undermined, because in the end, Greed is worse. Also, Greed destroys itself.
But wait! Emperor Palpatine is too major to be a minor character, so let's look further down the food chain. Remember the two (because once is not obvious enough, apparently) sequences in Phantom Menace where large fish creatures try to eat the submarine with Our Heroes? Other fish creatures come along and eats them, and again, allowing Our Heroes to escape. The fish creatures acted as Greed, showing Greed's self-destructive nature. However, because the creatures didn't have any individuality, the two sequences end up being unintentionally funny.
In other words, the thing keeps a story-element character a stereotype is that the element is tailored to the story's "so what." A minor element character can surprise the reader; a stereotype never can.