In your opinion, what makes a good (fictional) character?

I’m looking for a little help.

I’ve been writing a story for quite some time, in which I have a few main, or prominent, characters. Most of them I absolutely love and feel I’ve got a great handle on. The problem is that I think I’ve fallen too much in love with my secondary characters and haven’t invested enough time in the main character—the hero. But he’s not exactly the “grand” hero type, which is the challenge.

I guess what I really want to know is if it’s okay to have a main character whose personality isn’t naturally exciting, and if so what other factors in a story would make this acceptable? But general opinions on what makes a good character would be much appreciated, as well. : )

(I can provide more info on my main character’s personality, if needed; but I get tempted to write a lot at first, so I’m trying to scale back and make this question as simple as possible.)