It's really hard to say. I'm sure the developers had something in mind as they wrote the storyline, but I wouldn't be surprised if they had similar debates (read: arguments

).
The only thing that really comes to me naturally is that the protagonist is somehow human; being Gorion's adopted child, Imoen's sibling, and the fact that your background, and indeed most of the dialogue and storyline, doesn't hint at any kind of racial prejudice leads me to believe the protagonist was simply intended to be human.
Vehemence has a lot of valid points about the pro being a mage or sorcerer. In fact, a sorcerer fits all the more since sorcerers have
some kind of natural talent for magic; maybe they've got a trace of dragon blood in their heritage, or some other inherently magic-using creature, or perhaps they were born in an area of wild magic, and it somehow infused itself into their being; or maybe they're the child of a god.

The fact that Gorion was a powerful mage also hints in this direction, as well as Irenicus' temptations.
On the other hand, based on a lot of your interaction with the common folk of Candlekeep, it's easy to assume that the pro is some kind of fighter. A lot of the intense battles also seem to indicate this; in BG1, Sarevok is virtually impossible to defeat with magic, and it's safe to assume that the pro was the victor, not his party. A couple "tough guy" mano-a-mano challenges throughout the games also lean in the melee direction.
It's fairly easy to tell what the pro is
not, or at least not intended to be: I very much doubt the pro is a druid; a bard is also pretty much out of the question; a barbarian doesn't really make sense, considering that you grew up in Candlekeep; a monk is a strong possibility.
Ultimately, the BioWare guys designed the game specifically so that this question would be difficult to answer. Just about anything is possible with enough imagination, though some more than others. Neat discussion, though.
[ 07-23-2001: Message edited by: TheHellion ]