I think there's some passive evidence that the PC on the "Right" during the character creation process defaults as leader during certain events. Most notably, that's the character who determines when Merchants will reset their inventories (although this is only really noticeable if you somehow get your PCs' experience counts out of "synch" with each other).
I've also noticed that, more often than not, auto-triggered conversations will start with this character - although it can be circumvented in many cases by creative placing (or by "detaching" them from your party temporarily).
