"Empathy" is being able to understand why a character is doing something, even if it's something unfamiliar to us in our actual lives (flying a fighter jet on Mars, burying a body, jumping off of a bridge). And there are a few really human motivations that make all of us tick: greed, love, envy, loneliness, ambition, debt, self-aggrandizement, loyalty --- that's by no means a full list, but you see where I'm going.
If we can give any main character, no matter how dark or bitter or full or rage or ennui, in whatever situation that character appears, an underlying motivation that comes from one of these "universals," we've given the audience a hook to hang empathy on.