I've been thinking about the concept of nobility lately. I don't yet have a definition of what makes someone or something noble, so I've been taking it from the other direction - I'm thinking about the characters that we do call noble and identifying their characteristics.
Nobility seems to entail a certain kind of acceptance. Noble people accept that things function a certain way, and that they have responsibilities because of that. Along with this acceptance, there is a certain pride. I do not know if it is pride that they have to (or get to) carry out their responsibilities, or pride in being part of the system as a whole.
When a person does things because they must be done (especially when they are unpleasant), that person is noble. They can easily become heroes if they must fight against others who threaten the system (like the evil wizard who kidnaps the princess and the noble knight becomes a heroic knight because he is maintaining the status quo of the kingdom, but happens to be vanquishing evil and saving a distressed damsel in the process).
Most notably, noble people are humble. They work hard; they don't complain; they don't boast; they always stand straight and proud.
Nobility is a very admired quality in people. At times, it seems so unrealistic that it fits only in fairy tales. And yet, I think it is a realistic quality, and wonderful stories can be told of noble people who remain noble through their tribulations.