Braig is apparently the watchman of Radiant Gasrden, most likely given the position by Ansem to keep watch and help protect the kingdom. Dilan and Aeleus guard the gates of the castle while Braig ventures out further from the castle, keeping an eye out for suspicious people and then relaying anything he sees to Ansem. This, of course, encourages his habit of eavesdropping.
Braig most likely eavesdrops and spies on someone with Master Xehanort throwing about their keyblades all willy-nilly.
As we've seen with many characters previously in Kingdom Hearts, those who do not have the keyblade desire it, usually just by seeing it (Riku, Jack Sparrow, Sephiroth, Marluxia). That or he overhears Mickey talking about it with Ansem and THEN sees Master Xehanort with it. Unless Nomura decides to trick us and make MX the king that visits Radiant Garden (he is, after all, believed to be the ruler of Castle Oblivion).
I think Braig falls under the keyblade's spell and kidnaps MX so he can either interrogate him about how to get a keyblade or so he can steal his keyblade. Terra goes to rescue MX (he is, after all, like Terra's father figure throughout the game) and beats Braig and frees Master Xehanort.
Now, at this point there are 2 potential outcomes. Either Terra beats the living shit out of Braig while trying to free Master Xehanort, OR once Master Xehanort is free he decides to exact his own vengeance on Braig and causes the scar and need for an eyepatch, therefore leading to Terra seeing the evil side of his mentor and choosing to fight against him.
