name of the red head girl

The Little Red-Haired Girl and Frieda are two different characters. The LRHG was never shown in the comic strips and has no name.

I thought her name was Heather.

Actually, I'm pretty sure that the little red-haired girl's name is Heather, and her hair is long and straight. "Frieda" was an incidental, not-often seen character that I think was created early on, prior to any episode that includes or makes mention of Heather.