I have agonized over this question, both as a book reviewer and an author. As a reader, I’m pretty good at pinpointing why I do or don’t fall in love with a book and what the book might have needed to make me love it. But some of the books that fell totally flat for me are huge bestsellers by beloved authors. I can only conclude that they’re bringing their target audience something that I’m built in such a way as not to be able to appreciate.
There are certain traits that may be more likely to get characters loved. Strong desires, motivations, conflicts. Emotional transparency. But beyond that, I think reader responses get very personal and can vary wildly. Some people like volatility and unpredictability; others want archetypal heroes and villains. Some readers don’t want narrators to be prickly or angry, especially female narrators. Others seek out that kind of book. There is no magic bullet; I wish there were!
I’ve found it invaluable to have in-person critique partners with whom I can discuss my characters. Reactions are very candid and spontaneous, and you learn a lot about why certain characters are beloved more than others. Even then, though ... I had a devious, twisted character my CPs really enjoyed. Not all editors have been so enthusiastic.
