While experienced HR representatives typically try not to divulge the complainant to the person complained against (PCA) during a workplace investigation, it often becomes apparent from the line of questioning. (That doesn't mean, however, that you shouldn't report wrongdoing!)
What people don't realize is that sometimes the identity of the actual complainant is surprising. For example, if sexually harassing comments were allegedly made by Person A to Person B in front of three witnesses, the complainant could be the person receiving the allegedly harassing comments, any of the witnesses, or anyone who was told about the incident. What a complaint is, is a suspected violation of company policy and/or the law.
Remember that you shouldn't be retaliated against for making a complaint in good faith. Therefore, technically, it shouldn't matter if the PCA knows the identity of the complainant. If you are at all concerned about how your company approaches this, ask general process questions before saying that you have a complaint.