Trainers, both tough and weak against your pokemon, that you have to defeat in order to get to the Elite Four's Pokemon Center and Mart. Better stock up on max potions and max revives trainers 'cause the gym leaders were just the first steps to being a master. The Elite Four are tough as nails, hard as rock pokemon masters of their own kind and you and your pokemon are in for a load of trouble if you aren't ready for them. Good Luck trainers, you'll need it.