I Don't Care Much for Him . . . But I Really Love Them!
Readers don't care about individual characters even half as much as they care about individual relationships.
I've known writers who spend all their focus and energy developing their protagonists. They'll pour their heart and soul into that one central character until that person shines off the page, full of distinct personality and desires and will . . .
And then they wonder why their books still fall flat.
The thing is, as interesting as a developed character might be, a character is never as interesting as a relationship. And I don't mean a romantic relationship. Or not solely a romantic relationship, anyway. By relationship, I mean the chemistry of any two characters played against each other.
But in order for this chemistry to work, all of your characters are going to need a certain amount of fleshing out. Your warrior-maiden heroine's sissy best friend? The relationship between her and the heroine needs to be full of unexpected twists! Maybe the sissy best friend demonstrates a moment of courage that shames the warrior-maiden heroine? What does that do to their relationship (and, subsequently, the drama of the story?) Your scampy hero's serious mentor? What if he secretly despises the scampy hero in his charge? What does that do to the drama of their relationship?
It's all about the play of characters against each other. Never focus all of your attention on any one character, no matter how much you love him/her.
So which relationships are working best in your story to drive the action and drama? Which relationships are a little stale? How can you liven up the tension between those two characters? Do tell . . . I'm always interested!