Builders are autonomous machines whose sole function is to build new structures and improve existing ones throughout the City. As such, they are responsible for its constant and chaotic expansion.
Background EditCenturies before the manga opens, when the Administration first lost control of the Safeguards, the also lost control of the Builders. Without any authority to control them or provide them instructions, the Builders have continued building the City since then, creating new and bigger structures as time passed. However, over time the Builders' programming has degraded, and most of them build entirely random structures with no intended purpose in mind. This is why the City is filled with bizarre, pointless features, such as staircases that go nowhere or straight hallways barred by a door every foot.
Most builders are non-sentient; much like animals, they go about their business building, and don't acknowledge other creatures unless threatened. However, some builders, such as those Cibo encounters after fusing with the Level 9 Safeguard unit, are sentient and in fact intelligent, such as the Dog Builder, which hid Cibo from Sanakan, fearing for her life.
Builders are the most robotic of the many races that dwell in the City. While most humans have some augmentations, and cyborgs are almost entirely robotic, builders (and their "cousins" the Exterminators) are purely mechanical. No two builders are exactly alike; it is likely that they create each other, perhaps from the bodies of other builders.