The bard-theory sound kinda good.
Otherwise, I would say mage or cleric. Candlekeep has the greatest libary in Faerun, so study the arts of magics would only be natural, especially since Gorion, your foster-father, was a (meant-to-be)powerfull mage.
Cleric is also an option, since most of the people working and living in the fortress were monks (of the none-fighting nature).
When they say one has never been outside the walls of Candlekeep, they might not have meant it literarly. I mean, you might have gone for a picnic 1 km outside with Gorion and Imoen. I belive "never been outside the walls of Candlekeep" just means that you have never travelled.
With that possibility, and the fact that Gorion was a Harper, druid might as well be a choise. He might have had many Harper-druid friends visiting that talked about nature and stuff.
Theif, sure, Imoen could have influed you.
Fighter/ranger/Paladin, you can have read about them in books and dream the whole days about being one. So that is also possible; you are not that skilled in the art of war when Gorion gets killed because of the lack of training.
As I see it, only the classes new for BG2 (Barbarian/Monk) does not fit in.
There were no monks that knew the martial arts in Candle Keep, as far as I know. And barbarian... who the hell would dream about being a moron that just throws himself into battle after battle.
Half-orc does not fit so good either, since you would have been kinda badly met by people in the Baldurs Gate region (they don't like orcs).
But mage and cleric is the most logical alternatives.
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We call him Bobby Corwen
