Saint's Corpse

Heavy spoilers follow!

The Corpse Parts (聖人の遺体) are central to the plot of Part VII: Steel Ball Run, and of influence in Part VIII: JoJolion.

Apparently ancient human remains; the Corpse Parts are first described as belonging to a Saint, miraculously self-preserving, and disposed to imparting miraculous (Stand) powers (in the style of certain religious relics).

List of the 9 Corpse Parts

 * Heart: May have granted Funny Valentine his Stand D4C when as a soldier he fell onto it in the desert as his company perished. Over his chest appears the word "conscience" in a pattern resembling that of the location of the other Parts in the country.
 * Left Arm: Merges with an unknowing Johnny Joestar in the Devil's Palm in the Arizona Desert / Monument Valley, granting him the Stand Tusk. Like the Right Arm, the hand has a hole in its palm. Later it is taken forcefully by Hot Pants.
 * Eyes: The "guardian" of the Eyes appears like a robotic Stand; invisible except where covered by sand, holding one in each hand. Each has the word "turbo" written around the iris, and allow their holder to see Parts in the possession of others.
 * Right Eye: Merging with his own, grants Gyro Zeppeli the Stand Scan.
 * Left Eye: Enables Diego Brando to retain the dinosaur powers effected him by Dr. Ferdinand.
 * Spine: Discovered by Lucy Steel at the end of the 4th Stage. The Spine doesn't react to her, but when Blackmore takes it, he momentarily percieves Jesus' presence, and he and lucy are transported 34 km away due to the spine's power. When Blackmore is defeated, the Spine fuses with Johnny Joestar. Later it is stolen by Hot Pants, leaving a small fragment in Johnny's body, allowing him to retain his Stand. This last part is eventually stolen from him as well.
 * Rib cage: Original owner unknown. Around the time that Valentine acquires it, his physique spontaneously and greatly improves.
 * Ears: Sourced in the spring by the Tree guarded by Sugar Mountain and its Stand, exchanged by Gyro with a pair of rabbit's ears, they merge with Johnny Joestar, to no particular effect. Reluctantly, in order to fulfil Sugar Mountain's rules and save Gyro, Johnny relinquishes them to one of the surviving Eleven Men; agents to Valentine.
 * Right Arm:Right_Hand.PNG Sourced in the spring by the Tree guarded by Sugar Mountain and its Stand, exchanged by Gyro with a rabbit's arm, it merges with him, to no particular effect. Like the Left Arm, the hand has a hole in its palm. Separating from Gyro as he transforms, in order to fulfil Sugar Mountain's rules and save him, Johnny reluctantly gives it to one of the surviving Eleven Men; agents to Valentine.
 * Legs: The legs were found within a wolf pup which Johnny had fed. Gyro and Johnny did not realize the wolf pup held them and it followed them due to the corpse's compulsion to follow those chosen by the corpse. It was eventually deduced by Wekapipo and Magent Magent that the wolf pup held the legs, due to one of the Eleven Men seeing the projection of Johnny's spine showing a wolf at the great lakes.
 * Skull: Discovered by Lucy Steel while hiding from Valentine. She perceives the skull located in a fireplace flue. Seemingly becoming pregnant when it merges with her, the skull's shadow is then seen against her stomach.

Function
The Parts often grant those merging with them Stand powers (in a function similar to the Bow and Arrow of the original timeline).

Parts react differently to different characters, as seen when Gyro and Diego each took possession of an eye: Gyro acquiring a Stand, Scan, with Diego retaining properties of a Stand effect used on him (the dinosaur powers of Dr. Ferdinand's Stand Scary Monsters).

As discovered by Valentine, the Corpse is utterly unique, existing in only one dimension in the multiverse; thereby establishing a "root dimension". In neighboring dimensions, instead of the Parts, the racers compete for other precious items, such as diamonds.

History
The Steel Ball Run race itself is part of Funny Valentine's scheme to find and collect the Corpse Parts. With his direction, the entire Corpse eventually merges with Lucy Steel, before separating and being carried to and locked in a vault beneath Trinity Church, New York City, by the 2nd Diego Brando.

After the conclusion of Part VII, Johnny Joestar retrieves the Corpse for a final personal use.

The Corpse Parts are increasingly implied to belong to Jesus; although this is never explicitly confirmed.

The Saint's Death
Upon Jesus' death somewhere on the Western coast of North America, a massive earthquake and storm tore the land, splitting his Corpse into 9 Parts, which were then distributed by various forces of nature eastwards, resting at the locations he indicated they would to Joseph of Arimathea.

Prior to the Steel Ball Run race
In the Devil's Palm, each member of the military unit Funny Valentine belongs to perishes except for him, having fallen unconscious onto the site of the Corpse's Heart, when it merges with him, saving his life and motivating his search for the remainder.

Steel Ball Run race
Funny Valentine reveals that he guided the planned route of the Steel Ball Run race with the aim of scanning the country for the Parts besides the Heart, which clues him as to their rough location. The race's manager, Stephen Steel, is only vaguely aware of the Corpse's existence.

The Corpse is discussed euphemistically as though belonging merely to a saint.

The competition for the Parts
Coming from Naples, Gyro treats the news of the existence of the Corpse very seriously.

Johnny Joestar wishes to collect the Corpse in order to raise himself from a perceived "negative" position as the result of his family misfortunes and personal mistakes. Johnny is willing to sacrifice the corpse to keep it out of misguided hands, though this doesn't sit well with Gyro as he believes the corpse should be preserved.

Diego Brando reads Valentine's intention and the power of the Corpse, resolving to use it to support his own fierce ambition.

Hot Pants reveals a deep desire to acquire the entire Corpse in order to help her absolve herself of what she sees as a cardinal sin in her past. She seems more aware that the Corpse must belong to Jesus.

President Funny Valentine holds a deep sense of patriotism and seeks to use the corpse's power to bring great happiness and good fortune to America, having formed the Steel Ball Run around the search for the corpse parts.

Sensing an obligation to defend the Corpse from possession by its other pursuers, Gyro and Johnny come into conflict with Valentine, his agents, Diego Brando, and Hot Pants.

When collected by various racers, many Corpse Parts generate clues as to the location of the next Part.

Blackmore, when handling the Spine, receives a vision of Jesus, shortly before he is killed.

While battling Axl Ro, Johnny Joestar also receives a peripheral vision and vocalized advice from Jesus.

The complete Corpse
Valentine's plot unfolds as he explains that the Corpse represents the ultimate power, existing in only one dimension in the multiverse, accessible only via his Stand, Dirty Deeds Done Dirt Cheap. The one possessing it, like the first person on a round table to take their napkin from their left or their right side, will be empowered to decide some aspect of the fate of the world. His intention is to use it for the enduring security of his country, the United States.

Lucy Steel, with Valentine's direction, eventually merges with the entire Corpse, beginning a metamorphosis that also lends her a Stand.

The Corpse's sealing
After Valentine's defeat, as directed further by him, the second Diego Brando seals the Corpse in a vault in the basement of Trinity Church, New York City, in order to establish "the center of the world" with Diego as its leader. Waiting, stating that "the Corpse is not [his]", Lucy Steel kills him.

Sealed away, it seems that the only ones left with the sure knowledge of the Corpse's existence and location are Stephen Steel, Lucy Steel, and Johnny Joestar. Stephen states his belief that the Corpse belongs to no person or nation and is safest sealed away in the vault.

Post-Steel Ball Run race
Johnny Joestar is anguished by the terminal illness of his wife, Rina Higashikata, and decides to forcibly retrieve the Corpse from its vault and exploit its power in healing her. Chased by U.S. government agents, upon his sudden death in Morioh, Japan, it is currently presumed that these agents returned the Corpse to its vault.

Trivia

 * The Corpse Parts appear on the stage for Steel Ball Run characters in JoJo's Bizarre Adventure: All-Star Battle. When all 3 Parts are collected during a match, the player gains a charcter-specific advantage.