Either spouse can file. It does not matter who is the Petitioner and who is the Respondent; neither designation affects how the case operates. If the parties agree, they may both sign the Petition by including a “Joinder” in the Petition, but this isn’t necessary. If only one person wants the divorce, they can file without the other one agreeing, but there are requirements about personally serving the Petition on the other spouse (or some other options are allowed in certain circumstances).
Washington is a no-fault state. The Petition simply says that the marriage is “irretrievably broken.” No other details are needed, and the laws to divide property and debts are without regard to fault. There are very limited exceptions to this regarding wasting of assets (for example, intentionally failing to pay taxes during marriage or wasting funds on a drug addiction).