Jupiter is the son of Orion Assante, the first Grendel Khan, and Laurel Kennedy, Orion's wife in his later years. Crystal Kennedy is Jupiter's step-sister.
Assante has himself impregnated with Jupiter, using the era's advanced technology, because Laurel is unable to have more children. He is born when Orion is very old, but Orion loves him in the short time he is still alive. Laurel Kennedy takes over the government of UNOW after Orion's death, saying that Jupiter is too young.
Major Story Arcs
War Child: On the Run as a Child
Laurel Kennedy leads the world for several years as Regent in Jupiter's place. Jupiter is kept under guard and in seclusion in the Dakota Black Hills, along with his step-sister Crystal. Laurel's plan is to keep him out of sight and out of power.
However, a mysterious figure in black, Grendel Prime, breaks into the compound and kidnaps Jupiter. Actually, Jupiter doesn't resist, and it turns out that it is more of a rescue. Grendel Prime drives a hover bike and keeps Jupiter, still a young child, in the side car. When danger rears its head, Grendel Prime slides a metal shield over the sidecar to protect Jupiter.
Grendel Prime takes Jupiter on a chase around the world to escape Laurel and her Intelligence Minister, Heath, who send multiple squads of Red Devil soldiers after them. Throughout this quest, Jupiter never says a word. He usually obeys Grendel Prime, but sometimes misbehaves.
Grendel Prime takes him south through the desert, then to Chicago, where Prime kills a bunch of thugs. During all this and through the next few months months, Jupiter sees horrific sights of violence.
They are next beset by some rednecks who want to rape Jupiter but once again Grendel Prime dispatches them. They go to New York, which has become a poisonous pit, and fight off zombies and sentry robots. Jupiter has to wear a mask to breathe in the toxic air.
They go south, to bayou country. Jupiter gets restless and falls into a swamp, where Grendel Prime has to save him from alligators. They meet a witch, who casts a spell to make Laurel think they are killed in an explosion. She finally has to tell the world that Jupiter was kidnapped and killed, creating a huge ruckus, since the world loved Orion and looked forward to his heir.
Jupiter and Grendel Prime hop on a freighter, which is soon chased by pirates. The freighter's crew has no chance to fight them off, so Prime puts a cruel plan into action. He kills some of the crew to confuse the pirates when they board. The crew then take the pirates by surprise. At the end, only Jupiter and Grendel Prime survive, and the horrified Jupiter has had yet another lesson in the survival of the fittest.
They ride their hover bike over the ocean to Africa and almost die from exposure. A tribe drugs them, but turns out to be friendly and lets them go. They battle another tribe and continue on.
Meanwhile, Heath learns that Jupiter is still alive, and continues to try to catch him, but he does not tell Laurel; he is trying to take over power himself. Crystal also does not believe Jupiter is dead; she has a psychic bond with him.
Jupiter wanders away from Prime when they are searching for water, and is captured by mutants. Prime fights them off, but is knocked out from behind by some rebels. They put Jupiter in prison. These rebels, led by Azif A Barouk, turn out to be loyal to Orion, and after interrogating Grendel Prime, intend to let Jupiter out. However, vampires invade the prison and abduct Jupiter, taking him to their home in Siberia. Here Jupiter begins to show off his autonomy and resilience. He stabs a vampire in the eye with a spoon, and later secretly saves a coal from a fire to stay warm. All his time with Grendel Prime is beginning to pay off. The vampires lock him up in an abandoned nuclear silo.
Kierch, second-in-command of the vampires, strings up Jupiter and plans to feed him to their leader, Pellon Cross. Luckily Grendel Prime arrives in time and saves him. Jupiter manages to give Kierch a good kick in the face on his own.
After they leave the silo, Jupiter speaks his first words in the series, confidently commanding Prime and the rebels as to their next move. While still a child, he has become a leader of men.
War Child: Taking Back the Government as a Young Man
Ten years later, Jupiter is now a young man, trained and respected by the rebels. They have made a plan to put Jupiter back on the throne. During this time, his "mother" Laurel has gone insane, and Heath has taken over as Regent-Proxy. Jupiter, Grendel Prime, and the rebels move back to North America, and run into Crystal in Alaska. The group splits up--Jupiter, Crystal, and some rebels go to the Dakotas to find Laurel, while Grendel Prime, Crystal's lover Susan, and some other rebels go to the capital for the main assault.
Jupiter and his team break in and confront Laurel directly. They show her a hologram from Orion, who says he always knew about her schemes. Meanwhile Grendel Prime's team takes over the capital and allows Jupiter to transmit a message to the entire world, announcing his return. He forces Laurel to admit who he is.
Jupiter is welcomed back with open arms by the world's populace, and his reign starts off well. Laurel and Heath are executed. Grendel Prime leaves upon Jupiter's coronation, saying he was only meant to protect Jupiter during his time in the wilderness.
Fifteen years later, Jupiter is assassinated. Shortly after, Crystal writes a book about Jupiter.
In Other Media
Grendel: Past Prime is an novel, written by Greg Rucka and heavily illustrated by Matt Wagner. It is considered canon within the overall Grendel mythos. It largely focuses on Susan Veraghen's search for Grendel-Prime after the death of Jupiter, but has flashbacks to Jupiter's reign.
Jupiter's sister, Crystal, schemes to get Susan out of the way so she can pursue a more profitable relationship with Aziz. She asks Jupiter to make Susan the head of security at the Dakotas, his private retreat, and he does so. However, neither Jupiter nor Crystal ever visit the retreat. After five years, Susan realizes that she has been exiled, and quits in disgust. Soon after, Jupiter finally visits the retreat, where he is assassinated in flagrante delicto by his sole female lover (he has mostly male relationships), Elena Hamek. She stabs him with a needle she hid in her arm.
Note that the chronology here is a little different; in Past Prime Jupiter is assassinated approximately 5 or 6 years after becoming Khan, while in the Grendel Cycle timeline he dies 15 years after becoming Khan.
After his death, the Khanate starts to fall apart, and the Grendel clans degenerate from honorable warriors to brigands.