I'm taking a stab on it again, be ready for a rough ride.

Who your character is, why is he a blank slate. Your character is noone special or at least wasn't, just another Mind Flayer thrall. In fact they were the janitor cleaning up after all the messy experiments, and bad tasting brains. You were noone special, your life, your thoughts, your hopes and dreams null and void by the power of the Elder Brain. Yet brief sparks of cognition awakened things in you, when your masters psionic grasp slipped for just fractions of seconds. Then it happened 'Thrall 10562h please report to the impregnation room' yes you were called to serve the Elder Brain in a higher capacity, you were no a buffet, a mother. Your destiny was to become a brief meat suit for a hungry, hungry tadpole. A tadpole that desired a touch of divine, or a sprinkle of fiend, a dash of penaze, a pinch of Old One, a hint of rough and tumble, so you were callled. Then things got harry, when Sally and her dragon came storming out of a portal, the ship that you had kept cleaned for so long was now a battlefield. And you the newly appointed meatsuit, caretaker of an innocent hungry baby tadpole were finally free of your psionic prison.

Long story short even though you seem to be without a past, or with out ambition, D&D is about theatre of the mind at times. They can be who you want them to be, until Devs decide otherwise, their past, their present, and future is up to you to decide. They seem bland or blank slated go with this, or change it maybe they had an over bearing parent that forced them to act a certain way, or controlled them and now their free they don't know how to act, how to be themselves. Maybe they were simple, and that tadpole changed them, and now they are forced to deal with everything in a new light. Maybe their just simply unemotional, uncaring about the world. That they simply thing this is minorly annoying, and all these people whine to much, and all the want is to sit back read a book, and simply exist.