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

Halley McPhearson is young, pregnant, and new in town. Shortly after her arrival, she asks Kate for stenciling advice on the baby crib she is making. Halley is Kate's last visitor before her death. She stopped by Kate's home to drop off a photograph of her newborn baby. Halley breaks down as she says goodbye to Ellen.