People lacking in confidence will usually be low-key by choice. The lack of confidence could be due to many reasons. Say, current bad times in education, career or personal life. Such people would therefore invariably appear harmless to others. Often this harmlessness could be mistaken as their being 'good-natured.'
Give them some good times. Boost up their confidence. Make them feel some buoyancy of spirits. Then make a check on their good-naturedness. How quickly does one's success get into one's head? How fast do the nails of conceit start growing with the rise in confidence? Does the person succeed in maintaining the same modesty that he had during his 'bad' times?
I have come across more than one instance of people betraying clear changes the moment things changed with them.
I am reminded of the saying that the true character of a person is seen when he faces bad conditions. I feel it is equally true if put the other way round.
Ah! Finally a simple blog! :D