After fighting Arwin the first time, Aura has to pick which of her allies to send out for the Pokémon fight. Before picking an ally, press X to enter the menu and feed a bunch of apples to your wounded ally(or just spend perika to heal them up if you don't have any apples). Starshine should be enough to let them win against the Avian, Jellyfish, and Mutated Hydrangea provided Aura's level is high enough.
Allies slowly gain experience. Each level adds some HP and +1 to their stats. Base rate is 1 exp per day. Pre-festival, you can bump that to 2 exp/day by collecting and selling 5 star metal to the blacksmith.
A better Vader example would be David Prowse's voice acting, since that was real and changed.