Sweeper

Sweepers are characterized by a robotic or armored appearance, with a muted indigo and cold grey base colors accented with bandages and splatters of blood. Their heads range in shape from standard humanoid to rounded, angular, and elongated, and their necks also vary in length. their faces are covered with metallic indigo masks, some may be broken

Their 'eyes' can have different placements on the face and shapes, for example: one vertical line in the center, or a large one in the center, a classic pair of eyes, or asymmetrical. They have no visible mouths.

On their backs, each Sweeper has a large 'test tube', or a pair of them, filled with a glowing red liquid

Their arms also often have glowing red weapons instead of hands, typically resembling praying mantis claws or scythes, though sometimes they are pick-axe-like. If the weapons are too long, the individual can move freely on all fours. Both arms are not always replaced with weapons; instead, they may have a single normal hand, or both at once.

Fighting Style
Sweepers never fight alone, with hordes of them always present at any given time, making them highly overwhelming foes. This was true in Library of Ruina, and further shown in Intervallo V: Nocturnal Sweeping.

Sweepers primarily make use of Pierce attacks with Wrath or Pride Sin Affinities, which inflict Bleed onto their targets. All their attacks heals HP On Hit, and their survivability is further bolstered by Skills and Passives that provide Damage Reduction and passive healing on any unit's death. Their kits feature en-masse SP healing with the passive "For the Family" that provides SP for each living sweeper, and "Night in the Backstreets" which provides 30 SP on entry at Wave 2 and beyond. Many of their attacks feature Unbreakable Coins, meaning that taking damage is guaranteed when facing off against the Waves.

They are also capable of "reviving" through Persistence, a status carried over from Library of Ruina, allowing them to nullify damage, recover from Stagger and regenerate 60 HP upon taking what would be fatal damage at an 80% chance. However, this effect can be bypassed when the Sweeper is inflicted with HP Healing Down or if the damage surpasses their maximum HP. If the Persistence effect activates, they will continue using Skills afterward, including Cracked Unbreakable Coins.

Bands of Sweepers are divided into "families", and address each other with familial or cordial terms such as brother, uncle, and neighbor. These families do not reflect literal blood relations, but are instead a collection of metaphorical labels, similar to Bloodfiend hierarchies.[12] "Mother" is a term exclusive to an individual, the one who is responsible for turning people into Sweepers. She appears to be the leader of their kind, with even the power to wage war.[12] Despite not being literal family, the Sweepers appear to harbor genuine concern and affection for one another, seeking to keep each other safe.

Sweepers have a set of rules they abide by, one of which keeps their sweeping exclusively to the Night in the Backstreets.[9]

The Sweepers' presence in the City is permitted by the Head itself, which may personally request their disposal service at times, such as when a Wing breaks a Taboo.[9] Sweepers are not actually mandated to perform exclusively at night by any kind of biological force, rather, that just happens to be their "role" in City life and a rule that they follow. Sweepers are actually capable of a variety of complicated maneuvers, including communication, intelligence, and arriving on a scene at any time of day (such as one group's decision to attend the Library in search of books).

Sweepers are created by their "Mother" using what they consider "food".[10] The transformation of a human being into a Sweeper involves melting into the red fluid which is considered the creatures' "body", a specially patented substance with approval of the Head.[11] This liquid is consumed by the Sweepers in order to move, sweep, and eat.[11] Thus, in order to function, Sweepers are required to refuel periodically by consuming people and melting them into more liquid.[11]

Sweepers live in dens, which may be inhabited by one family of Sweepers each.[12] While the form of these dens is unclear, they are described as having lockable doors.

Sweepers are humanoid creatures occupying the Backstreets that emerge from their nests every night from 3:14 A.M. until 4:34 A.M. in three waves,[1] during which they participate in the "Night in the Backstreets", a period of time in which no laws are applied to a populace besides Taboos from the Head and the Wing in which the said populace lives. During this time period, the Sweepers destroy anything left outside, whether it be objects or people. They absorb organic matter, human flesh in particular, and convert it to the red fluid that composes their "body", while simultaneously destroying inorganic litter. This also includes vehicles, with the exception of those recognized as residential vehicles by the City.[2] Sweepers are considered a regular and predictable component of the City, with some citizens accustomed enough to make use of their sweeping as a rudimentary disposal service, most often for crimes.[3] Sweepers also exist in the Outskirts, and are considered a particularly high threat to children wherever they roam. Sweepers may prey on, accompany, abduct, or treat children like their toys.[4][5]

Due to their immense population, fighting against the Sweepers is viewed as a pointless endeavor. It is also because of these sheer numbers and organizational force that Sweepers are considered as much of a threat as they are.[6] Even if a Sweeper dies, the crowd will continue to march on and sweep up what lies ahead of them, including the fresh corpse of their fellow Sweeper.[1] Fortunately, the three waves of Sweepers each approach at predictable times, and someone can confidently consider themselves safe after surviving all three.[1] This march is described as an eighty minute sweep from one edge of a Nest to another, without rest.[7] However, the existence of a Syndicate, Association, or Office on a particular street is said to make it more difficult for Sweepers to raid.[8]

Sweepers are strange beings known to inhabit the Backstreets of every District in the City, only coming out at a specific time of night to destroy anything left outdoors, "sweeping" detritus away.

They can also be found in the Outskirts.