Thank you for playing! The personalities work like natures in Pokémon, where certain stats will grow faster and others will go slower depending on which one you have. Sticking with the default or using one that seems appropriate for the class will usually give good results, but I decided not to explain that fully in-game so that new players would let the personality options spark their imagination rather than feeling bogged down trying to make the best choice statistically. The difference isn't very big until high level, anyway.
I encourage you to play further, and offer up any ideas you may have on how to improve it the game!