>>54942829His entire rivalry is founded upon his desire for Ogerpon and losing her to the player. He laments his inability to fight for what he wants, and resolves to get much stronger to defeat you. It's revenge. It's incredibly easy to see, and even trailers for the Indigo Disk still frame his desire for revenge against the player.
Nemona starts as a Champion, having already reached the goal. Her desire to battle stems from wanting a friend, as her postgame story shows. Maybe if you had played the game you'd understand that.
