He's courageous because he takes on Robinson's case even know he knows there is no way he can win. He statnds up for people's right no matter if society criticize's him for it.
"I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do."
- Harper Lee, To Kill a Mockingbird