'one might expect' is an interesting way of putting a tyrannical theocracy with elements of mind-control. The harmful stereotypes work both ways.
I think if you want to be the exception to the rule, it has to come from somewhere more than your backstory. Otherwise, why involve the canon at all, or other players for that matter.
I'm.... not quite sure what you're getting at here, to be honest. All respect, but something has been lost in communication here, to me. All other things being normal in a world of established lore, if you find yourself assaulted by a furious Githyanki brandishing a silver sword that bursts with necrotic energy every strike... the name you would most likely expect them to be calling out, if they are calling one out at all, would be Vlaakith. That is what one might expect in such a situation. It would be surprising and unusual if they were calling out dedication to a different higher power. Being a paladin at all is also unusual, since they have a fairly rigid rank system, but in the case of an outlier who is dedicated to the lich queen and gains the power to smite foes as boon from her, and must have class levels, what would you call them? It doesn't much matter in this case - the example was that they were a paladin of Llira, after all - the extremely unusual factor was that they as an individual were dedicated to someone other than Vlaakith... which is unusual and unexpected.
I'm really not understanding the latter sentence either... Anything at all that is a part of anything that has happened to or around your character before the point of adventure - anything at all that has set them up to be different, unusual or exceptional, is backstory. It quite literally cannot 'come from' anything else... because anything that creates that exceptional circumstance
Is backstory, by definition. How those elements fall in with or stray apart from the relevant world lore are also a part of backstory, regardless of which they do; what is relevant is how those elements have affected the person your character has become up to the point of beginning your current adventure.
So, I do apologise, but I feel as though I must have misunderstood or failed to grasp what you were getting at or trying to say there, because the comment just doesn't make intelligible sense to me. I am sorry, that's not intended as an attack, just a note of confusion.
