While a character can have any number of virtues, there are a few that are nearly universally essential. These break down into two categories: Capability and Good-Guyness.
If your character is capable, he can get things done. The main virtues in this category are:
- courage (the character acts, even when scared)
- cleverness (the character can figure out what he should do), and
- a sense of responsibility (the character doesn't look for somebody to pass the buck to).
Your character may have a hard edge and some serious character flaws, but the reader has the sense that, deep down, he's just a good guy. For me, for me, is comprised of two things:
- fair-mindedness (the character judges all others equally--though not necessarily kindly or respectfully)
- compassion for the weak (the character will not stand for seeing the little guy beaten down)
There are exceptions to everything, of course, and each of these qualities has a well-known and much-beloved character who lacks it. But, in general, I believe these virtues are the cornerstone to building a character readers can love.