Secret Advisors are Imperial Advisors who are appointed by the Emperor, and confirmed by the Prime Minister, who are designated with all the power and authority of an Imperial Advisor. The difference is that their identities are kept secret. One such advisor was Sylvos Dawnlighter who was appointed to oversee diplomatic efforts outside the Empire.
Secret Advisors are appointed and are only revealed upon the retirement of the Emperor who appointed them, or when the Emperor chooses to acknowledge them.