Washington eviction law is written for landlords and tenants. The adult child who moved back in, the friend who came for a few weeks, the ex-partner still living in the house after the breakup: none of them signed a lease or paid rent, so the fast unlawful detainer process may not apply. File it anyway and a judge can dismiss the case months in.
Ejectment under RCW 7.28.010 lets anyone with title and a right to possession recover the property through an ordinary civil case in superior court. It takes a little longer to start than an eviction, but it fits the facts, it can include damages for the months of occupancy, and it ends with the same writ and the same sheriff.