The Bride is a humanoid Monster kept in the Facility. She is listed on the Whiteboard and is bet on by the Digital Analysis department. She is not seen during the System Purge.
Physical Description and Attributes
Since the Bride is not actually seen at anytime during the film, her appearance is unknown. However, its likely she resembles a young to middle-aged woman in a stereotypical white wedding dress, veil, etc. Since she is kept inside the Facility, its likely she possesses some outwardly or supernatural abilities and/or traits to help participate in sacrificing victims for the Ritual. Little is known about the Bride or her life before the Facility, but it appears something happened around or based off her wedding day that changed her into the person/monster she is now, hence her name. Her husband/groom/partner is unknown. Like Kevin, while she is never seen directly, her associated item and name feature prominently in the film, leaving the audience free to imagine her attributes and backstory.
In the Cellar, she is almost summoned by Jules Louden, who almost puts on her beautiful gem necklace which she found on a displayed wedding dress, possibly similar to the one the Bride herself wears. However, she is distracted by Dana Polk who is reading from Patience Buckner's diary.
The Bride is not shown at anytime during the film. The only way we know she exists is because she is listed on the Whiteboard, and her wedding dress, necklace and veil are seen in the Cellar and Jules almost summons her, as stated above. She does not appear in behind the scenes footage or in the visual companion.
This monster could be a reference to "The Bride" trope of horror films such as Bride of Frankenstein, Bride of Chucky, Bride of the Creature (from the Black Lagoon), the Bloodsplatterd Bride, the Bride from the Disneyworld attraction Haunted Mansion, or Gorgeous' Aunt in Hausu. A number of horror films titled "Bride" have also been made over the years featuring female antagonists either with or without supernatural elements, including 1973's Bride, The Bride (2017) from Russia, as well as others. Thus Cabin's bride could be a reference to one or all of them.
The Bride might also just be a satirical stab at fear of commitment during or before marriage.