Character class certainly is an element of the persona, but so are other factors such as alignment, personality traits, family associates, cultural background, specific event history, other natural talents, etc. that can be worked into the story.

If you have BG II and Shadowkeeper editor, change Viconia's class from cleric to thief and play through the game ... works just fine. None of her backstory and dialog interaction really depends on her class at all. In fact, she makes an excellent sneaky thief.