The Lass basically evolved into the character Blue (also known as Green depending on region), who in turn became the character leaf (the playable girl in leafgreen).
She's in the pokemon special manga as a main character, leaf she's playable, and then supposedly that girl pikachu runs under the skirt of in the anime into is her.
Here is old artwork related to the subject:

i'm not sure on the date of the image of her with a pacman ghost, but some of the stuff it's with is VERY old (pre-pokemon, back when game freak was a fanzine):

of course I have nothing to back it up data wise (ie no overworld sprite or something, i'd love to know it there indeed is), but it at least had crossed their minds back then.
here she is from the manga for the record:

and the lass artwork from the time:

the skirt girl in the intro:


assuming that the first image in my post came before the anime intro, i would be willing to bet that blue and lass are indeed seperate ideas that existed at the same time, but of course without real dates to these images that's hard to say, and even with dates would only be speculation. looks to me like the skirt girl is a lass recolor, but I'm not sure that means she is somehow the same character as Blue.