ADVISOR MANDRAGORA, The Moonlight Maiden
Mandragora is a rather mysterious character, atleast to the public; having been all but dragged into Equestrian Society by Aurora Solaire as the exiled Princess returned, Mandragora quickly cemented herself with being disturbingly knowledgeable about the goings-on within Equestrian politics and royalty and was thus given the role of Advisor. She still resides in the chamber within the depths of the castle, even long after Helios' rule came to an end.

She's built a little posse for herself, including the orphaned Elpis, her zebra spouse Asase, Jade's homonculus Emerald Eclipse, and eventually many of the Changelings that slowly integrated into society (The latter was largely in order to lighten the load on Jade and the castle staff; when your wife is a Changeling Queen, it makes sense that you'd get help from anywhere you can and Mandragora just so happened to be willing).

Oh, I should address that; Mandragora, despite the... very familiar horn, is not actually a Changeling. Many have speculated about her true nature - and due to her rather hush-hush nature about her personal life, rumors have only grown (especially when Carapace became Equestrian Queen and- yeah it's a lot to chew on) - but none have received a concrete answer beyond an ambiguous relationship to the ancient Perigee Royal Family, who'd been long since exiled from Equestria as the Solaire Family took control...

Eh, I'm sure it's nothing.