I'm on vacation, so I'm rewinding some of my favorite editing posts:
Here are the ideas that occur to me or that I've used myself:
1. Love something passionately Your character is selfish. They don't have any friends or very few. They're bitter and closed off. Give them something they do love and care about. A famous example of this is Katniss Everdeen from the opening of The Hunger Games. Deep down you know she's not a very sympathetic person. She thinks about her and her family and nobody else. But she loves her sister and so do you. Who couldn't love her after the duck thing with Prim's shirttail. Just make sure the audience also loves this person or pet. Yep, pets are great for this.
I've used this myself in Kindar's Cure. Kindar can't get along with her hateful family. She argues and speaks unkindly to her younger sister. To protect herself she must stay closed off and not reveal her feelings. But she loves her royal staff and does what she can to shield them from her mother's wrath. And she is also fond of her older sister.
2. Act of kindness Your character is a jerk. They're not very thoughtful. Like the shirttail episode between Katniss and Prim, have your character do an unnecessary act of kindness. They're awful to anyone and everyone, but they hold the door open for an old lady. They stop a kid from running into the road. They give up their seat on a bus to a handicapped person. They pick up some litter or do the dishes at home. They do something that makes a reader consider this character may be redeemable after all.
I've used this also. After a fight with her family, I have a character do the dishes as a goodwill gesture. Small sure. But maybe just enough when combined with the other tricks listed here.
3. Experience doubt/remorse Your character argues constantly with their parents or your character does has an immoral job- assassin, spy, thief. Have them experience some doubts about their choice of action. Put some question into their thoughts. "Am I doing the right thing?" Have them feel bad about fighting or being disrespectful--at least in their own thoughts--even if not voiced aloud. Maybe they still believe they are in the right, but they don't feel happy about their actions.
What could be more human than to have doubt and remorse. We all do it and can connect with it. Even as she fights, I had the character in number two feeling bad about the arguing. She stands by her belief that she's right, but she's not happy to have hurt her family.
4. Make them interesting If you can't beat 'em, join 'em. Your character is evil brought to Earth and has no good qualities. So make them fascinating. They do the unexpected. They keep the reader guessing. They have a compelling and attractive voice that stands out in a crowd. They'd make the cover of Time with their sheer unpredictability and gotta see what they do next.
5. Make everything else interesting In the vein of the last idea, your main character is a putz that nobody is going to like, but your world and plot is going to keep people reading. You make your world building so new and unique that nobody gives a crap that they hate your character. Or your plot is so nonstop that there's no time to pause in reading to consider if they like your protagonist.
Give readers something always new and fast paced and hope that's enough for them.
6. Paint them in a corner Your main character is no Mother Teresa, but you put them in such danger or into such a tight place that readers are instantly rooting for them. Get readers hoping your character makes it out alive and you've won half the battle.
I also did this in Kindar's Cure. I hit Kindar with one terrible thing after another in the opening chapters until she was literally forced to flee her home.
7. They ain't the worst Hand and hand with number six, goes this tip. Your character is unsympathetic but all the other character are even more distasteful. Your character looks good by comparison. I've seen this done a lot in darker stories. I'm guessing this is a GRRM tactic.
Many of my main characters seem to start out as not the most reader friendly. So there are the ideas that rattle around in my noggin or that I've employed without even thinking about it. Consider using one or a combination of these for your own characters.
Or maybe you have your own tricks and tips. What have you used or noticed being used to make a character likable?