A Non-Examiner
An independent chairperson isn’t there to examine you at your viva. They’ve not read your thesis and don’t have questions. They’re there to make sure the process is fair and make sure that the viva is following regulations. They might steer your examiners or provide guidance but they’re not there to examine you.
If you have a supervisor present at your viva then they’re not examining you either. They’re there to watch and maybe make notes. They can’t respond on your behalf and they can’t ask you anything.
If you want to know more about the examiners and non-examiners who might be at your viva then look at the regulations well in advance. Get a sense of what to expect, what that means for you and what you might need to do as a result.