The Reaper is the identity of three different Batman supervillains.
Dr. Gruener was a Jewish man who was placed in a concentration camp during World War Two. Many years later, Dr. Gruener took up the identity of The Reaper to kill Colonel Kurt Schloss, who was the former head of the concentration camp he was imprisoned in. Though Gruener successfully killed Scloss during a halloween parade, he ran afoul of Batman and apparantly died during their battle. It turned out many years later he was saved and cryogenically frozen by Black Mask. He has recently been thawed out.
Judson Caspian was a wealthy man whose wife was killed during a robbery. He then adopted the identity of The Reaper and began killing criminals in Gotham during the 1950s. After a battle with Green Lantern Alan Scott, Caspian moved to Europe. He returned to Gotham many years later to resume his vengance seeking as The Reaper. Being defeated by Reaper, Batman was forced to team up with Joe Chill who was a mob assasin that killed his parents. While hunting Reaper, Batman loses control and attacks Joe Chill, however Reaper killed Joe Chill first. Batman fights the Reaper again and wins. However he is suprised that the Reaper was Judson Caspian, whose daughter Rachel was about to marry Bruce Wayne. Judson jumps to his death rather saying Bruce will make a fine new protector for Gotham. Rachel becomes a nun after this and does not marry Bruce.
Joe Chill Jr.
Joe Chill Jr. saw his father's death at the hands of The Reaper, however since the Reaper killed himself he goes after Batman instead. Chill Jr. becomes the Reaper with help from his sister Marcia. Rachel Caspian, who believes The Reaper is actually her father come back to life becomes a target of Chill Jr. Chill Jr. captures Batman and uses hallucigenics to make batman relive the tragedy of his parents death to try and get him to commit suicide. Marcia turns on her brother and Chill Jr. kills her. Meanwhile Robin arrives and helps Batman defeat Chill Jr.