From SuspireWiki

Unusual in that it is both an official and unofficial position, the Harpy is, at its simplest level, a member of the “Kindred elite.” Harpies represent a who’s who of Kindred affairs, the celebrities and fad-starters. Other Kindred look to Harpies to see who’s in and who’s out, and what positions and opinions are popular this season. A well-known Harpy can sway public opinion faster with a biting comment than some Princes can with a solid decree. Harpies who attained their influence and position unofficially, through nothing more than personal power and charisma, are often more respected than those who are appointed by Prince or Primogen. An unofficial Harpy probably calls things as he truly sees them, whereas an appointed Harpy is usually considered a shill or even propagandist for the Prince.

Five Things a Harpy May Do

• Encourage Kindred to host social events (...so they can critique them later)
• Convince society at large of one Kindred's rise in popularity (...or loss of popularity)
• Put in a good word for someone (or help repair a broken reputation), for the appropriate price, with the Elysium regulars
• Aid an up and coming person (who pays well and promises reward) to subtly start the undermining of support for a Prince (...to pave way for a new one)
• Casually forget to record or erase boons (...for a price)