Lightning scars!
To my knowledge these are usually pretty temporary. Though I imagine with how often Jessie and James get zapped, theirs are a bit more severe/long lasting. They cover them up with makeup in the places where the scars would show.
Ash's go away quick. They're usually on his hands because he has, willingly given Pikachu his hand to shock in order to help him think, get an idea or wake up (this is a thing he actually does in the show LOL)















