The existing pathfinder brace rules don't require you to specify anything about which target or which direction you're bracing against with a brace weapon. This is sort of silly – if someone charges you from behind, you can't swing around and still be braced.
So we will say, when you brace you pick a direction, N, NE, E, SE, etc. (or up; or down, if you're flying and bracing against a ceiling.) You get the brace benefits against any charging creature that moves into one of the three threatened squares in that direction (for reach-1) or five squares in that direction (for reach-2) from outside your threatened area. (It should be clear which squares I mean if you draw it.) The creature does not have to be specifically charging you, just charging someone.
For 3D charges the brace region is extended up and down by one square; bracing up or bracing down produce brace regions of all of the threatened squares above or below you by exactly your reach.
Note that the brace action is a type of ready, so this is not the attack of opportunity you get when someone moves out of a threatened square. If you have a reach weapon and the target moves on through the threatened square to get to you, you can attack them again as an AOO – even with the same weapon – but you will not get the brace bonus for the second attack. If you do this with a boar spear, you will lose the AC bonus from the crossbar when you pull it out to attack again for the AOO.