I agree with Chris2Balls (almost wrote 3Balls then
).
The problem is especially true of the main characters. They look just as general as the back up characters. Generally main characters need to be "larger than life" in SOME aspect. For instance, if you want them to be rednecks (as a random example), they should be REALLY redneck. Trucker hat, stains on the singlet, shotguns out the wazoo. If you want them mild mannered, they should be almost to a fault. This makes them stick way out in the audience's mind.
Take Metal Slug, for example. You've got these guys wearing NO ARMOUR, eye patches, bandanas, and the like, because they are so at peace with the chaos. The warzone is their home. They even start laughing when they blow up a tank the size of a house. This is fun for them.
Or take, say, James Bond. He isn't some robotic government agent, quiet and orderly. He throws quips at his adversaries, throws a cocked eyebrow even in the most tense situations. He doesn't have the odd romance on his travels, he sleeps with EVERY damn woman between home and the objective. He is larger than life, and becomes unforgettable.
You can do this with pretty much any main character. Harry Potter - he isn't just a wizard, he's a wizard of incredible talent. He is the only surviving member of his immediate family. He doesn't just attend Hogwarts, he's at the centre of all the major events. Then Ron Weasley is a complete flip. He's hesitant, not so talented, has a whole garble of siblings and stays out of the limelight. All of these aspects make him more than just a reserved character, he is LARGER THAN LIFE reserved.
Okay so those are kind of really random (probably not the best) examples right off the top of my head, but my point is, think to yourself "what do I want the personalities of the main characters to be?" Then try in every possible way to push that into the extreme, to make it larger than life. You can take to the point of being comically exaggerated, which you can keep if thats what you want, but there'll be a point where you go "okay this is too far" in which case its pretty easy to dial it back a bit until you get the right amount.
Give it a try! I think you'll instantly have more personality breathed into the game. Even try it with the zombies. They are the shambling dead, brainless and thirsty (thirsty?) for brains! Push that element to the extreme (metal slug 3's zombies are an EXCELLENT example of this).
Outside of those design considerations, I think your characters could do with a good deal more saturated colours. Especially the main characters. Something eye-catching so the player can find them at a glance. I would do the redesign of the characters before worrying about that, though.
UP to you, though.
