While you’re stocking your plot with characters (or, if you work the other way around, while you’re assembling the cast of characters who will generate your plot), there are some types you want to steer clear of because they will lose reader sympathy — not just for themselves, but for any characters who happen to be standing too close to them.
One is The Annoyingly Perfect Character. This character is good at everything, and is always on the right side of any issue — no matter what the normal side may be for his time and place. Dogs always like him. He can drive a stick shift without ever stalling at a busy intersection. He can cook an intimate dinner for two and not have the kitchen stacked full of unwashed pots and pans at the end of the evening. If the character is female, she can do all of these things and run a Fortune 500 company without ever chipping her fingernail polish.
Another is The Character Who Wins All the Arguments. This usually happens because he or she is also The Character Who Agrees With The Author. Readers get annoyed by this one in a hurry, especially when they start thinking that the author is deliberately setting the character up with debate partners who aren’t exactly the brightest bulbs on the Christmas tree. (Yes, Robert A. Heinlein, I’m looking at you.) If you’re going to be writing a debate, remember that even the wrong side is likely to have one or two good arguments going for them — be fair, and let them have those two measly points before your highly principled hero crushes them under the weight of a dozen stronger ones.
And a third is The Character Who Can’t Get a Break. This is the guy (or gal) for whom the line “if it weren’t for bad luck, I’d have no luck at all” might have been written. If he has a job, he will lose it. If he gets a job, it will be in a sweatshop, or a soul-destroying cubicle farm, or a seething office morass of backstabbing and bureaucratic corruption. His significant other will cheat on him; or, if faithful, will contract a lingering and expensive malady that will cause him to turn to a life of incompetent crime in order to afford the treatments. He will leave his only umbrella on the bus. The reader will begin to suspect that the author hates this character, and will secretly despise the character for putting up with such unfair treatment.
Don’t write these characters. Your readers will be grateful for it.