As far as Criminal Law, one who registers a Complaint with a Police is called a Complainant.

Later, when the same Police files a Chargesheet [based on that Complaint] in the Court, the Police then called as a Complainant and the original Complainant thereafter called a Defacto Complainant.

Otherwise, in a Private Complaint filed directly in the Court under Section 200 Cr.P.C. the Complainant remains Complainant till the disposal of the case.