Tenchi Muyo!
No Need for a Backstory!
For thousands of years, the Ruins Buster, Kagato, spread destruction across the galaxy in his quest for ultimate power. He'd already kidnapped his ex-teacher from the Royal Science Academy, Washu Hakubi. He demanded possession of her three powerful gems, which would add to the abilities of his Soja Gems. Kagato's craft was the giant battleship, Soja, powered by the Soja Gems themselves. Washu refused to provide her precious gems to the villain, and so he imprisoned her in the reversed dimension of his ship.
Eventually, she decided to make a container for the gems, to keep Kagato satisfied without providing him with their power directly. The ultimate genius of the galaxy created a daughter for herself, named Ryoko. Created as a splice between Washu's DNA and that of a creature known as the Mass, Ryoko was the container for the Gems. For a partner, Washu combined the DNA of the Mass with a crystalline creature, creating a being she named Ryo-Ohki. This being took the form of a powerful space battleship. They were handed to Kagato to use as he pleased. They ended up being the most feared space pirates in history.
Kagato's search for the Jurai Power continued, and eventually he ordered Ryoko to attack the planet Jurai itself. The planet was panicked, except for the prince of the Royal Family, Yosho. He retaliated against Ryoko, using the sword, Tenchi-Ken. The battle raged for quite some time, but Ryoko eventually lost. She retreated to the planet Earth, where she doubted Yosho would follow, since it was a forbidden planet. The prince did follow her, and finished what his opponent had started. The power of the sword was enough to remove Ryoko's gems and embed them within itself. The pirate was so weakened, Yosho managed to seal her within a cave. He decided to stay there, in th event that Ryoko ever managed to escape her prison.
Washu, still trapped in the reverse dimension of the Soja, could not be forced to create another Ryoko. She'd never agree to it. Instead, Kagato used the knowledge he'd gained and created a child of his own to pirate the galaxy for him. He spliced his own DNA with that of the Mass, creating a young man. After that, a duplicate of Ryo-Ohki was created, though more Mass than crystal. Kagato was unfeeling towards his new son, and so neither of his creations were named. To try and make up for the lack of Washu's Gems, Kagato infused his son with power from the Soja Gems before sending him out to conquer planets, just like old times.
In a hundred years, these new beings had shown their abilities on countless worlds, and yet their percentage of the Soja's power didn't stack up to the power of Washu's artificial family. Kagato was furious, and exiled the pirates from their only home, the Soja. They spent six hundred years looking for someplace to belong, finding only resentment and hatred from the planets of the galaxy because of what they'd been forced to do. The only place they enjoyed spending time near was Earth, because no one there knew they existed. They hid in Japan, sneaking around and learning about humans. They soon learned the language of that country, and used its words to call each other by. Kagato's son named himself Oni, and named his counterpart Yoma.
Seven hundred years after her imprisonment, Ryoko was freed from the cave she dreaded by Tenchi, Yosho's grandson. The two princesses of Jurai came to Earth, looking for their half-brother. A Galaxy Police Officer was sent to investigate the battle between the Juraian ship, Ryu-Oh, and the space battleship Ryo-Ohki. The police shuttle crashed near Tenchi's home. Soon, Kagato himself came to Earth in search of the secret of the Jurai Power, the Ship of Tomorrow, Tsunami. Tenchi and his new family killed Kagato, destroying the Soja in the process. They returned to Earth and lived there together, the Ruins Buster all but forgotten.
What no one knew was that the Soja Gems, much like Washu's Gems, are indestructible. They spread across the galaxy after their ship's destruction. Silently, their power was hidden for over a year. It was only a matter of time before someone would find them.