Yes, if you're a defendant countersuing the plaintiff, you have to serve the plaintiff, as well as any other party that's appeared in the case.
Before you go any further here, consider hiring a lawyer. Litigation isn't easy, and there are a lot of rules and deadlines, and if you're asking something as basic as this now, take that as a sign that it will get a lot more complicated and you'll be over your heard very quickly.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Yes, the plaintiff has to be served. if you are in state court (Superior Court of California), service of the cross-complaint can be done by mail to the plaintiff's counsel. You do not need a process server to serve the cross-complaint on any parties who have already appeared in the case.
The preceding answers are correct as it regards parties that have appeared in the lawsuit. If you have named new parties in your cross-complaint, you must have a new summons issued by the Court, and personally serve [or some acceptable alternative method to personal service] the newly named parties with the complaint and any other pleadings that have been filed by you or the plaintiff.
You only need serve the cross-complaint by mail, with proof of service by an uninterested party, on parties that have appeared in the matter. Be sure to read the requirements found in the Code of Civil Procedure, the California Rules of Court and the local court rules. All of which can be found on the web.
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