Definitely not Shadowcat. She's a Claremont creation through and through. He was very fond of her and wrote quite a few arcs centered around her.

Here's a link to every character Simonson created: http://en.wikipedia.org/wiki/Category:Characters_created_by_Louise_Simonson

Now I'm curious as to who you're thinking of. Do any of these ring a bell? She did create the whole Power Pack family.