Under Washington law, you are responsible for serving the defendant, but the law does not require a particular person or entity. Anyone over the age of 18 can serve legal documents—a friend or family member, for example. Basically, anyone old enough can be your own process server.
There is one major exception. You cannot personally serve the defendant yourself … under any circumstances.
Should I hire a process server?
As I explained above, you are not legally required to hire a professional process server, but I strongly recommend that you at least consider hiring a professional process server. For three reasons:
First, unlike probably your close friends and family, the primary job of a process server is to “serve” (i.e. deliver) legal documents. It's their expertise.
Second, after the defendant is served, you'll receive written proof of service, signed under penalty of perjury. This document is called an affidavit of service. It may be a crucial document if the defendant challenges service later.
Third, the whole process is incredibly simple and can be done entirely online. Take ABC Legal, for instance, a well-known nationwide company. You prepare the summons and complaint, upload the documents, fill out a brief form with the requested service address and basic information about the defendant … and that's it. The company verifies your request via email and confirms delivery of service via email. You never have to call the office or communicate directly with the server.
In Washington, a good process server costs about $125, but it may cost you more depending on the number of attempts.