Think back to the characters you've met through books over the years. More than likely, the ones you recall are not the nice, even-tempered, straight-and-narrow characters. You remember the villains, the guys (and gals) with an edge, the ones teetering just this side of sanity, or the quirky, even volatile, people.
They caught your attention -- they made an impression. And a lot of the time, though you may not want to admit it, you could identify with them.
Remember that as you develop and write your characters.
We're all a little crazy, a little flawed. People are not black and white. Though some hearts may be a bit blacker than others and some souls a shade whiter, basically, we're all some hue of gray. And there's a wide range of gray in-between black and white.
Readers can resent a character who is too saintly. Yeck. Unbelievable. Uninteresting. They can also turn away from an antagonist who is too evil. Yeck. Unbelievable. Get real.
Nice, sane, likeable characters need a touch of insanity, something to make them interesting -- to keep them from being overshadowed by the counter character. And the bad guys need a likeable quirk or some almost redeeming quality or motivation to keep them from becoming cartoonish.
If you're not sure how to do this, check out books on psychology or abnormal behavior written for the lay person. You'll find personality models laid out for you. Look for books that profile personality types or the sociology of character.
Remember, though, that whatever *flaw* your character has, it will -- must either directly or indirectly -- affect the plot, the character arc, or the outcome. Your bad guy can't have the *flaw* of loving his mother while murdering innocents, without that bit of character likeability having meaning to the story or climax. Your protagonist shouldn't have the *flaw* of being rigidly judgmental of others without that crack in his armor being important to his development in the book.
Don't be afraid to write characters who are not like sane, centered, lovable you. Write the characters who are similar to the *other* you, the one you try to keep hidden. Now, I'm not saying write an autobiography. Just look inside yourself and admit that you're not perfect. And neither are your characters -- not perfectly good nor perfectly evil.
Characters who are edgy, flawed, or "gray" are more likely to be remembered. And if the characters are remembered, there's a strong chance the author will be as well.
5 months ago