In Washington, it is the entity responsible for prosecuting the case that files criminal charges. Generally in Washington there are three possibilites for who can file charges. First is the Attorney General. The AGs office very rarely files criminal charges, usually only in exceptional cases involving high profile cases. Next is the County Prosecutor. The County Prosecutor is responsible for filing charges and the prosecution of all felonies that occur within the county and for any misdemeanors that occur in an unincorporated portion of the county. Cities and the prosecutor for the city handle the filing of misdemeanor charges that occur within city limits. Depending on the City this can be "in-house" attorney, attorneys hired specifically to be prosecutors by the city (like Seattle and Tacoma), County prosecutors in effect hired by the city through an agreement with the county, or an outside attorney(s) hired to do criminal prosecution (like Fife or Orting).
The decision to file.
The prosecutor has wide lattitue to file. The RPCs, the Rules of Professional Conduct that control and guide every attorney in Washington, have specific rules that govern prosecutors. RPC 3.8 states: "The prosecutor in a criminal case shall (a) refrain from prosecuting a charge that the prosecutor knows is not supported by probable cause." Probable cause is a very low standard: whether under the facts known to the prosecutor is there a probability that the person committed the crime. This standard is below the preponderance of evidence (the 50.1%). Many cities and counties have higher internal standards, but those standards vary. The prosecutor will be reviewing the police report, the defendant's criminal history, the witness(es) statements and any other information they may have before wishing to proceed with the charge.
The witness effecting the decision to file.
Victims do not file charges. Only prosecutors do. Certainly the prosecutor can take into account what the victims wishes, but the prosecutor can, and often does in domestic violence cases, ignore those wishes and proceed with a prosecution anyway. The victim has no authority or standing to compel the court to ignore the prosecutor's filing decision and dismiss the case.