mm...RPG's... they are so easy to work with since you can let your imagination loose all over them...okaaay...lets see here...
To work with things like light vs. dark, and people inside your head you basically only need to sort one thing out and then work from there.
Why does he have a dark figure in is head? Why can he use those powers? Is it something that runs down his family? Maybe a magical artifact? Dark magic? Some kind of accident invovling magic?
There are way to many directions this story can take...
What if the dark figure is some kind of mage trying to control the main character? Maybe he is the killer of the characters family, and now haunts you? It could also be a sinister side of yourself like in the Prince of Persia: Two Thrones.
The magic might be from when he found/forged/was given his blade(s)? Maybe the mage trying to control him is accidentaly letting magic pour through the connection, making you stonger as the game goes on? Maybe dark and light magic can be refueled in different ways?
God I really wish you hade some more details
Well... if you make some specifications like.. Is he a royalty? names? enviroment? what kind of setting? etc. etc. and I can throw together a basic story summary and post it later.
I wish you all luck
//Devoto
Graviora manent...