How do I evict my girlfriend who has never paid rent?

The problem is how to classify her tenancy. There are 4 main types of tenancies in WA, Commercial Tenancy, Residential Tenancy, Tenancy at Will, Tenancy at Sufferance. An unlawful detainer action may only be used to Commercial and Residential tenancies. A residential tenancy requires that she pay rent. Obviously this would not be a commercial tenancy. If she is breaking into your house, you may be able to use a forcible detainer action, or possibly forcible entry. The other options are using an action for ejectment (takes a long time) or a trespass action. There are a lot of options and a lot of attorney opinions on what you may use and how you may use it. I think that you need an action for ejectment . . . unless she breaks in again, then forcible detainer. While it may take longer, it may save you money in the long run by selecting the correct cause of action. You really should hire an attorney to discuss your options with to do things the right (and cheaper) way.

Mr. Weatherstone is a much better source of legal advice than the police. That said, a tenancy at will is not one of the types that can be terminated via an unlawful detainer action. I would consider strongly whether you should be looking for an anti-harassment order. This is very fact specfic, and you should discuss this with a local attorney to get an idea on how your local court approaches matters like this.