A person who files a civil lawsuit against another party is called a "plaintiff".
In a court, the plaintiff is the individual or gathering who is blaming someone else or aggregate for some bad behavior. In case you're the offended party, you are guaranteeing that a law was broken, and you're in court to introduce your case.
The plaintiff charges, the respondent tries to demonstrate that allegation off-base. You've seen this relationship on network shows about legal counselors, or possibly you've been to court yourself.