My question involves landlord-tenant law in the State of: Washington

I've been living in the same apartment for 4 years and my roommate moved in September. I'm hoping to evict my roommate soon, he has caused a lot of trouble with the homeowners association, disagrees with how utilities should be split and it just isn't worth the trouble anymore. I'm tired of fighting and I would rather live alone.

My landlord knows about these issues. I am the only one on the lease, I have had permission from my landlord to sublet these past 4 years but I forgot to have him sign a sublease. Is he considered an at will tenant now? Can I evict him or will my landlord have to do it?