First level characters are competent heroes, but at the beginning of their hero game. Some may have experience in things directly relating to their class, such as a sage wizard or a soldier fighter, while others have experience elsewhere, such as the soldier wizard or sage fighter. Their backstory needs to make sense that they have the mechanical capabilities given by their class, be familiarity with armor, ability to cast spells, whatever, but there is no mechanical necessity that they can only pick backgrounds that directly lead into class. I can play the scrappy urchin fighter who learned to fight on the streets as easily as the noble fighter who learned fencing and intrigue at her mother's foot, as easily as the solider fighter, or any other combination that makes a cohesive character. Class descriptions are not prohibitive - they do not bar any ideas that aren't explicitly listed in them.