Who is Halley McPhearson from One True Thing and what is their importance?

Halley McPhearson is the young pregnant woman who moves to town and asks Kate for stenciling advice on the baby crib she is making. She also learns how to embroider headbands with Kate's help. She is Kate's last visitor before death, arriving to drop off a photo of her newborn baby. She has an emotional breakdown as she says goodbye to Ellen, the first person aside from family to cry about Kate's illness.