The metaphor is the story, not the character.
I think the Hulk has always appealed very strongly to much younger readers than Spider-Man, because Spider-Man is an adolescent character, and the Hulk is a very childlike character.
I'm not building each one character around one metaphor, so much as trying to build a heroic archetype that can be used to express the kind of metaphors that I find in each story.
The characters are, by their nature, archetypes that can serve different metaphors.
If there's ever a character who can only serve one metaphor, I'll probably tell one story with that character and be done with it.
At one point, I worked up a list of five requirements for a superhero: superpowers, a costume, a code name, a mission, and a milieu. If the character had three out of the five, they were a superhero. But that's just my definition.
Youve got to leave the reader with more than just a name and a costume - they need to know who the character is, what theyre like, what kind of attitude they have, what sort of role they play.
What really matters is not how well a character fits a definition, but how strongly he or she resonates. Characters with strong, resonant ideas at their core will have more of an impact on the cultural consciousness than a character who's just an empty collection of attributes.
Marvel's got a crowded universe, and there are already so many characters hogging the spotlight that it's hard to break through that. First off, whatever character you're creating, odds are, there's already someone similar in one way or another.
That's the way it happens - some characters you set out to use, some are happy accidents. As long as it works, it doesn't really matter how you got them.
I created lots of characters in high school and college, and the first character I created in pro comics was Liana, Green Lantern of M'Elu, for a backup story in 'Green Lantern #162,' my first professional sale.