I believe the most intricate plot won't matter much to readers if they don't care about the characters, especially in a series. So I try to focus hard on making each character, whether villain or hero, have an interesting flaw that readers can relate to.
“I'm always the one with the activist friends. I've been an activist very little.”
“Being in a wheelchair for 30 years. I'm not whining about it because I don't dwell on things I can't do anything about, you know. I never really think about until somebody mentions it. I did take a bullet.”