The petitioner to a civil court case is the person who first filed a petition and a notice of summons to start the court case. Similarly, the person to whom the petition and summons was addressed to is the respondent to the case.  

The petitioner and respondent to a case will continue to be the petitioner and respondent throughout the duration of the case, EVEN WHEN the respondent files a notice of motion. Be sure to remember this when filling out any paperwork that asks you to list the names of the petitioner and respondent.