Can a lawyer sign on behalf of a client? Does it have legal effect? In court, we found that the plaintiff was not present, and the opposing lawyer changed his mind at will in front of the judge.

The lawyer can sign on behalf of the client. If the disposition of substantive rights requires special authorization, that is to say, the entrustment agreement signed with the lawyer generally does not include the authorization signature for the disposition of rights, and special authorization is needed. In the case of special authorization, the lawyer may sign the disposition right on behalf of the client, and the signature is valid. in your case, the other party should give the lawyer special authorization, so the plaintiff can't be present, and the lawyer will exercise his rights on his behalf.