There are several excellent sources to help you choose your plants. Your county, King County, has a Native Plant Guide with plant lists and How-to articles. The Washington Native Plant Society has a page, Native Plants for Western Washington Gardens and Restoration Projects, with plant list categories which includes a list for Groundcovers: Evergreen, Deciduous. Here are suggestions from these sources for shade tolerant groundcovers with additional information from our Native Plant Database:
Achlys triphylla (Sweet after death or Vanilla leaf) and here is information from Washington Native Plant Society and from King County Northwest Native Plant Guide.
Polystichum munitum (Western swordfern) is evergreen and grows to 3 feet but could function in a groundcover area. Here is information from King County Northwest Native Plant Guide and from Washington Native Plant Society.
You can find more possibilities in Native plants for deep shade on the King County Northwest Native Plant Guide page.
Washington State University Extension's Gardening in Western Washington has links to publications, fact sheets and websites that you might find helpful.