The story of Jadeite's life, more or less ^_^ This information should be correct, even if I do indulge in a few speculations (but they are marked as such)
Please note that this biography contains spoilers for season one of Sailor Moon, as well as a few details from the fourth season manga, Sailor V and various flashbacks to the Silver Millenium.
During the Silver Millenium era, a magical kingdom existed on Earth. It was called the Golden Kingdom and was ruled by the royal family of Earth, whom were also guardians of the Golden Crystal.
The heir to this kingdom, prince Endymion, and his four guardians Jadeite, Nephrite, Zoisite and Kunzite were the assigned protectors of Earth. The guardians were commanders, lifeguards and advisors to the prince and their official title as a group was Shitennou, the Four Heavenly Kings.
Jadeite, who at 18 was the second youngest of the Shitennou, had responsibility for the eastern areas, which today makes up Asia.
Not much is know about exactly what Jadeite did during his years as a guardian, nor how he was as a person, but it is believed that he had a relationship with the Princess of Mars, which might have been something of a political problem, since relations between Earth and the nations of the Silver Millenium weren't the best. Except for a vague comment made by Jadeite and one single image, nothing is certain however.
What is verified on the other hand, is that Jadeite's master and the princess of the Moon, Serenity, were a pair. Sadly, it was this couples love that marked the beginning of the end.
A young noble-woman by the name of Beryl also had a certain interest in the prince, and liked to spend her time spying on him, hidden in the shadows. When he turned her down, she became more and more bitter, and the last straw came the day Beryl spotted Endymion together with Serenity
Somehow lady Beryl came into contact with the demon Metallia. A child of Chaos, created in the storms raging on the sun, Metallia filled Beryl's heart with hate, and twisted her appearance into a monstrous beauty. Then, proceeding to fill the hearts of all people of Earth with hate and envy at the Silver Millenium, Metallia and Beryl began to take over the Earth.
The Shitennou also fell under this spell of hate, and turned against their master.
Accusing the Silver Millenium empire to spy on them, and claiming that the Earth was oppressed, the Shitennou demanded that prince Endymion should stop seeing Serenity and join their cause.
However, Endymion was the sole person who managed to withstand Beryl, and escaped to the Moon, hoping to warn the people there so they wouldn't fall into the same trap as his own planet already had.
The Shitennou, now leaders for the armies of darkness, lead a successful attack on the great palace on the Moon. With the help of the demon Metallia Beryl and her new commanders managed to completely ruin the Silver Millenium.
Prince Endymion died in battle, and filled with grief princess Serenity took her life. Upon seeing this, Queen Serenity made use of the holy Silver Crystal, banishing Metallia and killing Beryl and the Shitennou.
Fast forward to our time...
Four young men meet up, having a feeling that they should search for someone or something. But before the idea is put in motion, a woman named Beryl approaches them, and bids them to join her under the darkness.
Still not clearly remembering who they are, or why they have come here, the men join her. She implants small control stones in their foreheads, the same stones they're named after, and uses these devices to brainwash them.
The Shitennou have returned, but they are yet again acting as warriors of the Dark Kingdom.
The newly awakened kingdom has one great need: energy. The first who gets the mission to collect it is Danburite, a servant of the Shitennou. After he is destroyed by a soldier going under the codename Sailor V, Queen Beryl orders the Silver Crystal to be brought to her, since it is the only thing that can sate Metallia's needs.
Jadeite is the first to go, his mission being both to search for the crystal and, until it's found, collect human life-energy. When sending out a youma in the guise of Naru's mother, Jadeite encounters Sailor Moon. Beryl became severely displeased with him, since he had neither gathered energy nor found the Silver Crystal.
But as always, Jadeite has a new plan. Deciding to use the energy of young scholars, Jadeite sends out a youma to the Crystal Learning Center, with instructions to brainwash the pupils through their computers. Again, the Sailor senshi show up, and this time it's not one but two warriors that fight together and destroy his demon.
By now, Beryl is severely displeased, and decides to punish her servant for failing. Trying to apologize, but seemingly accepting his fate, Jadeite bows and prepares for the inevitable, when Nephrite interrupts.
The King of Stars laughs and mocks Jadeite, he even asks if Beryl can't give him the mission, before asking what exactly the holy Silver Crystal is. Maybe realizing that knowing what one is searching for is helpful, Beryl explains the power of this mystical crystal, before ordering Jadeite to destroy his Sailor senshi. The implications are clear... destroy them, or I'll destroy you.
Counting on the Sailor warriors's habit of always appearing where he least wants them to, Jadeite this time creates a plan with the purpose of drawing them out. Disguising as a bus driver, he begins to abduct young women, with the intention of feeding them to Metallia. Until the soldiers have come to meet him, he 'stores' the potential victims in a sub-space of some kind.
Time passes, and reports of young girls having vanished as they take the six o'clock bus from Sendai Hill are increasing. A young priestess living in a nearby shrine gets disturbing visions of a man hurting her new friend Usagi, and goes out to stop this approaching evil. But as she sees the bus, the driver draws her in, and she is abducted.
Sailor Moon sees the bus, and manages to climb on it just before it enters the dark dimensional gate. The bus goes deep underground, and as it fades away, a castle appears.
Jadeite is meanwhile fascinated by the beautiful priestess, Hino Rei. Feeling that something is a bit off with her, he admires her beauty, maybe even remembering something from the past...
But as Sailor Moon and Sailor Mercury approaches, Jadeite ignores whatever feelings he has, and uses the girl as a shield, holding her in a choke-hold.
Sailor Mercury attacks with her fog, but he merely laughs and turns it into ice, letting it fall to the floor. Becoming more confident as he sees how easy it was to ignore this attack, Jadeite catches the Moon Tiara in his bare hands when Sailor Moon attacks. That was a grave mistake, as Luna manipulates the magical disk and it twists out of his hand, surrounding and catching him. But still, Jadeite isn't helpless, and he uses his powers to blast the Sailors into a wall.
With her last powers, Sailor Mercury throws a transformation pen to Rei, who uses it to become Sailor Mars, the warrior of fire. With her magical flames and a holy parchment, she burns Jadeite to death.
As the Sailor soldiers leaves, it's shown that Nephrite and Zoisite have watched the entire fight, and as a furious Nephrite sees his friend being killed, he swears revenge on the warriors of theWhite Moon.
The body of the first king is then put into a glass coffin, and we see Nephrite visit it, saying that he will use the holy Silver Crystal to bring Jadeite back to life.
The three remaining Shitennou fight, and one after another they fail, leaving only Kunzite alive in the end. As everything comes closer to a conclusion, and Sailor Moon activates the crystal, it is revealed to the kings that they once served prince Endymion of Earth. But as the crystals light goes out again, the three kings turn back into stones.
When Kunzite is later killed, a brainwashed Endymion takes these stones, and keeps them on him during the final fight. And then, when Queen Beryl and Queen Metallia are both destroyed, the truth about the stones come forth.
They hold the souls of the Four Heavenly Kings.
Now finally freed of evil, the kings can serve and protect prince Endymion in spirit form, until their souls can be reborn.
Jadeite was born approximately 20-25 years before the fall of the Silver Millenium. Nothing more is known about his past or origins, but it's a safe guess that he originally came from Earth and had some connection to the other Shitennou before they joined the side of evil.
Beryl, a commoner or low-class noble woman, was the leader of the group which would later become the Dark Kingdom. With the help of the demon Metallia she awakened jealousy and hatred in the people of Earth, making them more then willing to turn their own planet towards darkness.
She also got the Shitennou to become commanders and generals in her new army, and soon all of Earth was under Metallia's rule. The only person who had the strength to withstand Metallia's influence was prince Endymion.
It's quite likely that it was during this time Jadeite got the title Commander of the Eastern Division forces.
Once Earth was taken, the Shitennou led the dark armies into the heart of the Silver Millenium empire, the Moon itself.
During this assault on the Moon is the only time we see Jadeite with weapons in hand. He's carrying a sword and a round shield.
With Metallia's magic the kingdom fell quickly, and both princess Serenity and prince Endymion died. Beryl proclaimed that the Dark Kingdom, with her as queen, now ruled both Earth and the former Silver Millenium.
Queen Serenity, devastated by this loss, used the Holy Silver Crystal to banish Metallia and all who served her. The armies of Earth were destroyed. The power of the crystal nearly managed to kill Metallia, and tore queen Beryl and the Shitennou apart. Finally, the newborn Dark Kingdom was sealed in a sub-dimension.
It is unknown how Beryl and her commanders survived, or what happened to them the during the thousand year period they spent locked away. But after a millenium in the Dark Kingdom, they managed to break free, and were now in dire need of energy so Metallia could be awakened from her sleep.
Jadeite volunteered himself to gather life-energy from humans. His way of collecting energy was to lure people to buy some magic items or to gather them at a certain place, and then draining their life-force.
Jadeite seemed to be very good at organizing, and his jewelry sale (and radio show, and pet shop and... you get the point) worked good, until Sailor Moon showed up with Tuxedo Kamen in row.
The sailor-suited defender of love and justice always tripped his plans, and this did not make Queen Beryl glad (understatement of the year). Things became even worse when one of Beryl's favorite demons, Thetis, who had something of a crush on Jadeite got killed during their plan to drain energy from couples.
So, the Queen finally ordered Jadeite to stop fooling around and kill Sailor Moon, or else...
Jadeite created a huge illusion over Tokyo, and threatened to set the whole town on fire, unless the Sailor warriors would come and meet him at the airport for a final showdown.
Sailor Moon, Sailor Mercury and Sailor Mars came. Jadeite first waited until he saw them transform and so finally found out which their real identities were he. After that, he tried to crush them with some airplanes. All in all, it worked decently; the Sailors were running for their lives, afraid to destroy the jets since they were so expensive, and Jadeite just had to wait until they were crushed. That's when Tuxedo Kamen showed up again.
He and Jadeite started to battle in the air and finally they fell into the water. Only Jadeite emerged (but Tux-boy slipped through a plot-hole. Damn!) and began taunting the Sailors, calling them weak little girls. Wrong thing to do.
The Sailor warriors became angry and when Jadeite let the planes attack them a second time, they were ready for him. Sailor Moon acted as a decoy, giving Sailor Mercury enough time to fill the runway with her fog. Jadeite landed on the ground, and Sailor Mars threw one of her holy ofuda on him. Somehow, this made the jets change course, so now they all headed straight for... you guessed it, Jeddy himself.
After being run over by one of the planes, Jadeite escaped back to the Dark Kingdom, where Beryl waited. She was furious that he had lost, and been forced to flee. She didn't bother to hear him out, and therefor missed the chance to learn the names of the senshi, but instead put him in Eternal Sleep. With a final scream of horror, Jadeite was frozen in the crystal. And that was the end of the first of the Heavenly Kings.