I think if we talked about some of our favorite characters, and why we liked them, each of us would cite different reasons, but I also suspect that there may be an underlying principle or two that explain why we like specific characters, in general.
Admirable Virtues 102-6
Audiences need to like the protagonist(s), to both relate to and admire them. People often relate to characters that remind them of themselves; characters who face similar types of problems. People often admire characters that demonstrate a skill or proficiency, particularly if that skill demonstrates altruistic motives, such as teaching or medicine, or if it has a strong visual appeal, such as performance arts or athletics. Intelligence is another admirable trait, though when it comes to brains a person can be smart, knowledgeable, or clever.