How could Rudy be a sorcerer before he was an adventurer?
Landers (or People of the Land) do gain levels, but is very low compared that of an Adventurer.

As per his wikia:


  He put tremendous effort into this guise; as a Lander, his EXP leveling rate was 1/4 that of an Adventurer, yet he was able to achieve a class level similar to beginning adventurers before his status change