I’ve been wondering the same thing

I KNOW THE ANSWER TO THIS. She is a Lass from the first generation trainers that you would encounter in the game, and the reason why she’s in the opening credits, is because the original Japanese opening theme song had a weird inappropriate joke in the lyrics about the lengths to which the protagonist would go to catch all the Pokemon

“(I’ll get you, Pokémon!)
Even if in fire, in water, in grass, in forests
In ground, in clouds, in that girl’s skirt (eek!)”

You can see Pikachu running between her legs to seal the joke. 

@citricbear

@rosexknight

OOOH OKAY