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 don't do well around the angry, bitter and emotionally fragile among us, which may eliminate 70% of the population.”
“I don't want to feel like a failure to my daughter. She's the best thing I've ever done. Buffy - pretty great and all, but Charlotte's way better.”