The musical yokai Rin Satsuki exists only in unused game data of "The Embodiment of Scarlet Devil", the first Windows Toho game. She never makes an appearance in that game, or anywhere else in the series, for that matter.

The Toho project fangame "Ephemeral Unnatural Balance", currently in development, promotes Rin Satsuki from apocrypha to main character. The game team is led by ExPorygon, who is the director and programmer. The character illustrations are by Sixten (myself).

A demo video of the game is at https://www.youtube.com/watch?v=i8B5o4RS3tU