Jack is the sin Wrath, and the Grim Reaper. A soul burdened by much sorrow and grief at his task, he doesn't remember the sins he committed at the beginning of the story.
He was a green anthromorphic bunny in life, and now only his head looks very recognizable, the rest of his body made up of a flowing brown cloak. He possesses the ability to manipulate his fluidlike body and fly. He normally carries a scythe, but many of them get destroyed or lost. He is skilled with his scythe, somewhat less with his bare hands, and intensely powerful when he gets angry. The level of his anger is shown in his eyes, which go blacker and blacker until entirely full when he is at his worst anger.
Jack was born in a literal test tube, spending the first years of his life developing inside of a tube of liquid. The scientists noted that at five years old, he was at a ten-year-old's development. One of the scientists there was Kane, then Dr. Kane. He lived there for a while, and was introduced to the concept of God , who had nothing to do with his creaton. This and listening to another scientist complain about God disillusions him about the Almighty. Later, he tried to escape and see the outside world.
He was brought back after being caught and expressed confusion on seeing a naked lady. The scientists revealed that at an early developmental stage he was castrated to prevent him from reproducing. He noticed that they wouldn't have done so with only one of him, and he is introduced to Jill, who he then becomes very close to. He was introduced to another rabbit experiment named 78, who would later be named Fiver by Jack. They became friends.
Later, Jill was killed after one of the scientists raped her. He pleaded with the scientists to bring her back to life. They saod they could not, causing Jack to realize they are not gods after all. He became homicidal and angry. At a press conference, he announced he was going to kill everyone in the room. Through undetermined circumstances, he later lead an army of furries to exterminate the humans. During this time, he was lovers with Central and an unnamed skunk , but his devotion to Jill broke both relationships apart.
During the chaos of the war, Central was killed.This event caused what little empathy he had left to vanish. He sent nuclear missiles all over the world, killing off everyone and everything. Fiver furiously stormed in, telling him off and then letting him know the skunk had been killed by Grimm. He then killed himself with a poisoned drink, thinking of himself as a god. He saw himself as the sin, Wrath, right before he dies. His last request is to not remember any of it. He caused so much damage that a metaphorical "reset button" was hit, and history began again, with him now locked into the role of the Grim Reaper.
Unlike the other sins, Jack cared about the souls he leads to judgement, but could not remember anything about his life. The comic started with him leading Fnar into the afterlife. He showed a different side when dealing with the merciless teenager killer Brian, gleefully sending him to Hell to be dealt punishment.