Nature is how other people perceive a character. It is not a set attribute which your character has to follow, but rather a variable attribute that can change over time. As the game progresses and the character lives his life he will grow and change. A backstabbing thief could reform and become a pillar of goodness, while a honourable, good man could undergo a traumatic experience and become a bitter, homicidal maniac.
When creating a character, the player decides what their nature is. Any descriptive words could be used: good, evil, neutral, loud, cruel, honourable, whiner, does not play well with others, rude, lazy, unpredictable, and the list goes on and on.
Every time a character gains a stage, the referee has the option to change a characters nature. Remember, nature is how the character is perceived. If a player says his character is good, and then goes about killing little old ladies just for the fun of it... well, you get the idea. The bottom line is that nature is how the referee thinks that non-player characters are going to perceive the player character.
Obviously, nature is also used to describe non-player characters to the players.
Some characters are able to determine another beings nature easier than others. Anyone with a stage of Diplomat or Leader, or any seasoned character (50+ skill) can determine, almost immediately, the nature of anyone who isn't hiding it. For characters without these skills, it will take time to get to know the being.