When entering Polomolok Town for the first time, Seth will come to the player(hero) from behind and ask if you're one of the trainers chosen by Prof. Oak and he'll say to the player that he is also chosen by Prof. Oak to save Southmin from chaos, and he'll battle you.
When entering Gen. San City(after receiving Oak's Parcel), You'll enter the scene, coming towards you and say that if you are really chosen by Oak, then battle me, and the hero will battle your character.