I've read about this figure, that supposedly was in the car with the Emperor and had the duty to whisper "you are not a god" just in case he forgot. Is this true, or just a myth?
Not just the Caesars but anyone having a triumph. It's a holdover from the republic, discouraging anyone from attempting to seize power. The exact words that were said changed based on the person and time period. It's discussed here:
https://reddit.com/r/AskHistorians/comments/2v4u35/did_the_romans_really_have_peopleslaves_whisper/