Courage

A kerbonaut with a very low courage

The courage of a kerbonaut determines how they will react upon specific events occurring around them. If for example part of the ship explodes those kerbonauts with a high courage will not begin to flail and yell as they usually do, but will instead laugh it off. It is closely linked to the stupidity value, and since they appear to be like a scale (high stupidity, low courage and vice versa), it can be assumed they work the same way.