I have been doing some work on character development for my novel. I found that though I can write pages about the main character, other characters, who have an important role and will be there for most of the story(ie, they are not minor characters), only get a small paragraph.

So my question is: Other than the hero, should I do some work trying to develop these other characters as well? At the moment, they are just 2D figures, with little personality.