The Borealis (ボレアリス Borearisu) are a group of WATER Fairy-type Spirit monsters used by Leah in the Yu-Gi-Oh! ZEXAL fanfic, Yu-Gi-Oh! Delta. They are mostly angelic humans in appearance, that are illuminated just like the aurora.
The Borealis creatures live in the North Pole. More accurately, they live in two different worlds; the world of the Spirits and the real world. They act as guides and embassadors to Spirits that wish to cross between worlds. The portal for this transition lies hidden within the aurora, and the Borealis Spirits can close it at will when none wish to cross. The world on the other side of the portal acts as a safe haven for these Spirits.
The main rulers of this spiritual crossroads are each refered to as a Goddess of Borealis (ボレアリスの女神 Borearisu no megami) and they 'watch the watchers', so to speak. They prevent unauthorised trespass to/from either realm, and make certain that the Borealis Spirits carry out their duties.
The Borealis' dueling style is very much similar to that of any other Spirit Decks. However, they are much stronger searchers, and therefore can carry a great deal of hand control. Sharing the WATER Attribute gives them access to support cards that regular Spirit Decks cannot enjoy, such as Salvage, Umiiruka, Moray of Greed, and can even delve into Synchro Summoning capabilities with Fishborg Launcher.
Since they also share the Fairy-type, their possibilities are increased further, as they gain access to cards like Victoria, The Sanctuary in the Sky, the Herald series, and Archlord Kristya (which is especially potent as all the Borealis monsters are Spirit monsters, and thus have no need for Special Summoning).