Torwards the end, they show Charizard, Blastoise, Venusaur, Ash, and then some girl with purple hair...Who's the girl with purple hair? I've always wondered that.,

She never appears in the show, but she does look like a lass from the games. Now that were on the topic of here, there is a very naughty moment with her in the opening.

I think that girl is named Blue. There's a pokemon manga where it's based off the Gameboy games and there are three trainers who resemble the characters in the game. The character who looks like Satoshi(Ash) is named Red, Shigeru (Gary) is named Green, and that girl in the introduction just might be Blue.

Her hair is just a little too poofy to be Blue though. And yes, Pikachu runs under here and her skirt goes up a little bit and you can see something.