Different car companies can place different geographical restrictions on where their cars can be driven. You would probably have to inquire of the particular company from which you would be renting.

Would this be a one-way rental, or would to ultimately return the car to the pick-up location?

Assuming that you are otherwise eligible for admission into the U.S., driving a Canadian-plated vehicle from British Columbia into Washington State should not be a problem at all. And once you are in the U.S., you may freely drive across state lines.
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