A Sweeper (掃除屋, Sōjiya) is one who dedicates their life to catching criminals and receiving rewards for their capture.
"Sweeper" is just a slang term for a bounty hunter, one who hunts criminals for their bounty. Certified by the Government, they get their jobs by finding wanted posters and or pamphlets with information regarding the target. These posters are usually put up in bars or other areas Sweepers tend to frequent.
In some ways, they are compared to mercenaries, who also go after bounties, but a different kind; they kill their targets, while Sweepers capture their targets and avoid killing. Despite their "no kill" policy, Sweepers tend to duel to the death with each other.
Jobs vary in difficulty, depending on how dangerous the criminal is, ranging from "F" to "SS". A fairly weak criminal is rated as an "F" rank job, while an extremely dangerous criminal, such as Creed Diskenth is given an "SS" rank.
The Sweeper's Alliance was an organization created by the Chronos Number, Lin Shaolee, in an attempt to capture and eliminate Creed. In the anime when Eve was captured by the Zero Numbers, the Sweeper's Alliance banded together to rescue her, alongside Chronos.
Informants are the peole who usually aid sweepers in tracking down their targets. They also keep them updated on important information.
- Eve is only an assistant to Train and Sven, until she is 15 and therefore old enough to get a licence to sweep. The age restriction likely acts to keep deluded untrained youths from getting hurt.
- A common practice with sweepers is using themselves as bait to lure out targets in hiding. For example if the target only goes out at night and everyone stays inside, the sweeper simply needs to walk out u the open to lure their target.
- Sweepers seem to get paid in cash for their bounties, as Sven is never seen visiting a bank during the story to withdraw funds.
- As the job requires a lot of travelling, sweepers may not have permanent residences; Sven notes his has a few hideouts to stay at (to save on money). In the anime, its implied Saya actually lived in the same town as Train, as she does not leave after catching Preta Ghul and is quite knowledgeable about the fireworks festival.
- Despite the bounties literally saying a sweeper can capture a target Dead or Alive, throughout both the manga and anime, Sven constantly complains that they lost a bounty every time a target dies.
- The term sweeper likely comes from the phrase "sweeping under the rug" for how the sweepers clean up the streets by catching criminals.