Erika has a butt.
When God expelled Adam and Eve from paradise, He used His holy sword of fire to slice the bottom part of their torso, thus creating the butt. After that day, they were forced to survive in the human world and eat unholy food full of impurities that they had to shit out of their anuses.
The butt crack is a mark of shame that has been passed down from generation to generation. That's what "human" means. Erika is a human and therefore her butt is split in two. Pokemon do not have a butt crack or an asshole, because they are not lowly humans.
Erika's ass. The most shameful part of her body. The part where Erika's back loses its noble name.