Yes, your lack of criminal history will go a long way to help you in this case. Should you plead guilty or be found guilty at trial, your sentence will be lower than if you had previous history.
Anyone who is convicted or pleads guilty to a felony assault in Washington is subject to the Adult Sentencing Guidelines. The more points you have (from previous convictions or other current charges) the higher your score. You sentence is determined based on this score so the higher the score, the higher your sentencing range will be.
In addition, someone with no history is far more likely to get a favorable offer (possible reduction of charges or deferred sentence) to resolve the case. I do not know the details of your case and would advise against discussing it on any public forum such as this.
Finally, since you claim you were not involved in the assault and that the person who did it is willing to take responsibility, you may want to take this case to trial. I would advise consulting with an experienced criminal defense attorney as soon as possible to discuss your options. Most offer free consultations.
Do not count on your friend stepping up. Many people say that; once they talk to their own attorney they often change their mind. Your lack of a record will help you, but you need to tell your public defender everything and consider doing what you need to do to protect yourself. Good luck.
The response I have provided is general in nature, and does not create an attorney-client relationship. My practice is based in Rhode Island, and the law and practice in other states or jurisdictions may be different.
You need an experience criminal defense attorney now. Get one and talk over your options and needs. Witnesses need to be talked to and interviewed because you don't know what people are going to say at this point in the game. Is there video from the bar?