Baba Yaga
Baba Yaga is the most famous figure of Russian folklore and mythology, hands down, being the only character anyone who isn't Russian can probably name. A hag's hag, Baba Yaga varies wildly between different stories from standard child-eating ungodly abomination to a chaotic trickster mentor, who can be both ally and enemy depending on her mood and how much respect you show her. Some of the few things that remain constant; she flies in a mortar and pestle, she has a sweet-ass house that moves by walking along on giant chicken legs, and you do not fuck with Baba Yaga.
Baba Yaga has warranted occasional mentions in Dungeons & Dragons; for example, the infamous witch of Greyhawk, Iggwilv, is rumored to be one of Baba Yaga's daughters, whilst she was instrumental in creating the Demon Prince Kostchtchie.
However, it is in Pathfinder that she has truly shone, as Pathfinder has no shame about stealing from real-world myth and folklore to pad out Golarion. Baba Yaga is a huge plane-travelling "Mover and Shaker" type figure, despite coming from humble stock as a Middle Ages Russian peasant girl whom a powerful faerie taught the arts of sorcery for shits 'n' giggles. She is the star of the "Reign of Winter" adventure path, which lays her entire history and power set bare in its final issue.