Okay, remember how I said you needed to wait on developing the plot before worrying about characters? The reason is simple: It’s easier to make characters when you know where they will fit.
Think about that for a moment. When you create the characters first, you are stuck with the characters that you have created. You’ll almost feel guilty about throwing a character out. If you find that you don’t need a character, you’ll try to write that character in, no matter what damage you will do to the plot. On the other hand, if you wait until the plot has been developed, you’ll know exactly what characters you need, which makes designing them a bit easier.
Another consideration is that by looking at the plot, you’ll see some characters jump out at you as needed that you hadn’t considered. Even better, you’ll be able to see where you need minor characters that you didn’t think about when designing the characters in the first place.
Keep in mind that you can always change the plot; it’s hardly immutable. However, the plot gives you an excellent skeleton on which to place your story. If you decide that a minor character would be fun to have a romance with, or otherwise need to give him more lines, so to speak, go ahead; plug into the plot and see what happens. If nothing else, the romance can either end up helping later on, thus bridging a potential plot gap, or add a more organic feel to the comic.
By setting up the plot, you’ve also decided how dark or light the story will be, and your characters can be designed accordingly. And, since your plot only dictates the basics of the characters, you can design full backstories for them. Had you designed the characters first, then you would have been meshing in all sorts of strange backgrounds without much care how they really connected.
Oh, and don’t be stuck by some silly arbitrary number when you create characters. It’s easy to say that you need X characters per Y pages; that’s hogwash. Just realize that there is a balance that needs to be struck: The fewer characters you have, the more you can explore their backgrounds, but they also need to be far more capable (after all, they’ll be needing to do more). On the other hand, if you have a larger cast, you may not be able to explore their backgrounds as much, but you can also have more fun, you don’t need to worry about consistency as much, and you don’t need massively able characters (which means that the combats won’t last as long).
In short, by doing the plot first you save yourself some grief later on. And that can be a great thing when you have some world-spanning epic you are debating…