Seelies are also known as faeries, or the Fair Folk. Pixies, nixies, and elves are also considered Seelies. They cannot lie, though they use evasive and distraction tactics to avoid directly answering questions they do not want to. They are an extremely manipulative, cunning, and mischievous race of Downworlders, their angel and demon blood attributing to both their beauty and malevolence.
They are led by a queen. The Seelie Glade has entrances scattered throughout the world. Their most guarded secret is a dimensional doorway that leads to an alternate dimension. The Seelies are its protectors, and the doorway can be opened by a Seelie at the point where the dimension intersects with the Seelie realm: between a pair of trees at the remotest edges of the Glade.
Seelies are in tune with nature. Seelies live among them, and the Queen has power over even inanimate beings in her territory. They can also harness the power of the earth and procure weapons from it. Also, most Seelies have tattoo-like markings of leaves or vines on their faces which can be manipulated somehow by the Queen.
The Seelies are generally allied to the Shadowhunters and comply with the Accords. However, with the return of Valentine Morgenstern in 2016, after the Seelie scouts requested by the Clave to search for Valentine were killed, the Queen cut off communications with them. According to Meliorn, the possibility that Valentine may win the war against Shadowhunters, as well as what they deem to be the unfair treatment of the Fair Folk under the Accords, may be the reason the Queen decided to cut ties with them, albeit temporarily. Some Shadowhunters felt that hope would be lost in the war against Valentine if the Seelies switch sides.
- Seelies apparently do not speak to vampires much unless they have to. Some may even consider them enemies, though if it rivals the antagonistic relationship of vampires and werewolves is unknown.
- In their culture, if Seelies are surrounded by butterflies, they are in mourning, and their knights wearing Seelie armor signifies that they are at war.