A Fairilu Door (フェアリルドア Feariru Doa?) is a magical door with a link between the Human World and the Fairilu World. These doors have different designs and can be unlocked with a corresponding Fairilu Key.
In the Fairilu World, there are three kinds of Fairilu Doors that are related to a Fairilu's growth: the Door of Birth, the Door of Growth and the Door of Dream. The Door of Birth is the door Fairilus have to push open it in order to born from their Fairilu Seeds. The Door of Growth is the door Fairilus use to find their own dreams for the sake of their future, meaning the door that appears when Fairilus are casting Fairilu Magic. The Door of Dream is for Fairilus to make their dreams come true by connecting to the places where can let them achieve the dreams with it.
The Fairilu Doors are also the final test of a Fairilu after being born. To mature properly, they would need to find the corresponding door that fits with their key and pass through it.
In the Human World, Fairilu Doors can be built through common materials. These doors can call upon a Fairilu once opened. To meet an actual Fairilu, it will need to be unlocked with its corresponding key and say the magical words: "Rilu Rilu Fairilu!".
Moreover, Fairilu Doors can connect to different places, and even worlds, such as Little Fairilu and the Human World. Fairilus can travel to various places or worlds (especially the Human World) by creating one of the Doors with their magic, or find one somewhere in Little Fairilu.