The Bar Raiser is a specially trained interviewer from a different team than the one hiring you. Their single job: make sure every new hire raises the average quality at Amazon.
Bar Raisers have veto power. One "Strongly Not Incline" from the Bar Raiser can override positive reviews from everyone else. They evaluate you against ALL Leadership Principles, not just -.
You won't know which interviewer is the Bar Raiser. Treat every round as if it's the Bar Raiser round. That's the safest strategy.