Can someone please help me? What do the defender, manipulator, statesman, warrior affect?
They affect only your stats: eg. warrior has more psy power and psy defense than aptitude and personality, they don't affect anything else. Basically, each class is just a set of predefined stats i.e. you don't have to choose one they're just sets of recommended stats.
Its best to do a custom one in which your personality and your aptitude is at 12
yulerruler: I suggest you look at the initial recommened character stats sticky thread in the TSoG walkthrough section of the forums to see which stats are best to start the game with.
Those given above are just fancy names for pre-set stats combinations.