Flush him out on the page, and that will help you develop your character to your audience and yourself. Give him a mission statement, only known to himself. Write down what it is that he is stubborn about, and what he keeps his mouth shut over. Give his mother a good reason for being a drunk and let your character struggle and be dynamic with this close relation.
If he's athletic, then he'll need to be involved in some athletics maybe on a team or in direct competition with another character. Pick a sport or two and make up your MC's daily schedule, which will need to include lots of practicing and exercise as well as time spent with sister, mother, and other main characters.
Give him a name! A full name, with some part hinting at symbolism will help you direct your character's flavor and personality.
What about his alignment? Never divulge this directly to your readers, but you should determine if your character is Good and Lawful, or perhaps Unlawful and Evil, maybe even somewhere in between like Unlawful and Good taking matters into his own hands for justice, or Lawful and Evil using the system for his own personal gain. He could be truly Neutral, a stoic mind that practices Buddhism or nihilism.
Good luck and have fun with your character and your story!
Source? Just my brains. You should check this out too: http://www.google.com/search?q=character+alignment