Honestly, both are horrible because Gardevoir because Gardevoir is a Special Attacker. Brave raises Attack and lowers Speed, which is the complete opposite of what you want. Serious is a Neutral Nature, which isn't bad, but isn't good either. A Modest (+ SpA - Atk) or Timid ( +Spe, -Atk) are the most desirable natures.
Based on the 2 you have, Serious is better because it isn't detrimental to Ralts, especially since it's in-game, but a Modest or Timid Nature would be better.