They are the fairies' friends. Whenever you free 10 of them, a secret place opens up for you!—Manual, Rayman Revolution
A Familiar Spirit is a collectible energy creature found only in Rayman Revolution. They are small, blue, glowing lights that have been imprisoned in cages by the Robo-Pirates. Each time ten Familiar Spirits are set free, Ly the Fairy's head appears in the game's heads-up display, meaning that she can now open a new Spiral Door which will lead Rayman to a new minigame. In order to access these eight minigames, Rayman must meet with Ly at one of three special locations: Ly's House, Ly's Stele and Ly's Cave. There are a total of eighty Familiar Spirits to be freed throughout the game.
There are some parallels between the Familiar Spirits in Rayman Revolution and the Tings in the original Rayman game. Both are sparkling, blue, floating collectibles, and while Ly lets Rayman access a minigame in exchange for ten Familiar Spirits, the Magician lets him access a minigame in exchange for ten Tings.