“Everyone is a moon and has a dark side which he never shows to anybody.”
Mark Twain has touched on the very essence of what it is to be human. Yes, we do all have a ‘dark side’ – that part of our nature that rarely, if ever, sees the light of day. We may well be acutely aware of this flaw but most of us are clever actors able to hide this face from public view. Jesus, in his criticism of the religious leaders of his day, called them hypocrites because they appeared as white-washed tombs which look okay on outside, but are actually full of dead men’s bones on the inside.
As writers we can tap into this deep well of darkness. We can let the protagonist have a flaw which other characters know nothing about. This dark side can be either a motivator stirring his resolve, or it can be a burden, an obstacle preventing success.
This character flaw can be shown in various ways: through actions, through internal thoughts, through uncharacteristic responses to other characters, through subtle hints which other characters do not understand and so on.
Possibly one of the best known example is that of Luke Skywalker in the Star Wars stories. All the way through the last three movies he battles his fear. On many occasions he is urged to give in to the dark side, to allow his fears to breed hatred.
It is certainly worth digging deep within this rich vein of conflict within the nature of your main character, exploring it and then exploiting it for all its worth.