Shinigami/Grim Reapers are the apparent guardians of the mortal world in the Soul Eater Universe. Contrary to popular mythology, the Shinigami of the Soul Eater Universe exist to prolong the lives of the living instead of cutting them short, as well as preventing Kishin from reappearing in the world.
(Shinigami is the Japanese tranlastion for Death God. The Grim Reaper was a figure who appeared in popular European Culture around the time of the Bubonic Plague, which was also called the Black DEATH.)
In the Soul Eater Universe there exist several Shinigami/Grim Reapers. First and foremost being Shinigami Sama or Lord Death. The second is his wife, Lady Death, followed by his son, Death the Kid, and his daughter, Hannah.
Lord Death founded an academy to train Meisters and Weapons on how to defeating Kishin, Pre-Kishin and Witches. This organisation was called Shibusen or Death Weapon Meister Academy (DWMA for short). This academy is staffed by several adapt Weapons and meisters. The most prominent being Professor FRanken Stein, Death Scythe Spirit Albarn, Professor Sid Barett and MIra Nygus.