I think that his parents will always be unknown. Like serenity's father...and the whole theory that Queen Serenity cloned herself..or Usagi is somehow the Imperium Silver Crystal..it's always a guessing game
Guys, Endymion's mother is actually from the Negaverse. You see this whole "Queen Beryl was once an ordinary human" rumor is just some P.R. from the Nihon department for people to feel pity for her. No, the truth is Queen Beryl is an evil being from the alternate dimension filled with humanoid demons, and she wanted to strike at the moon, but since she couldn't assassinate Queen Serenity directly, she decided to corrupt the Earth. She sent a Negaverse spy to seduce, then kill, the King of the Earth, and but the spy fell in love with him instead and gave birth to Prince Darien. That's why Darien is so easy to hypnotize and brainwash...he's half DEMON.*
No, I'm just joking around. I'll admit, while the idea of Beryl being Endymion's mother would make for some interesting fanfic, it doesn't fly in any version of Sailor Moon, even the North American dub.
*As convoluted as this version is, this has nothing on my actual fanfic ideas.
Oh gosh, you've done it now. I'm going solely by memory here, but here it goes...
The Story of Queen Gaia
Once upon a time there was a woman who lived in the northern regions of Earth. She was a common woman with a very common problem. She was in love with a man who did not return her affections. This man was from a faraway land and was strange in appearance to those around him, as his eyes were dark and his skin was swarthy. He was not rich -- in fact he was a lower in station than herself, if possible -- yet she only had eyes for him.
Unbeknownst to her, another individual by the name of Cubare had been watching her plight for some time. One day he introduced himself to her, and offered her his aid. He would guarantee the foreigner's lifelong love for her...for a price. In exchange, she would forfeit her soul to the Devil upon her death. Without hesitation she accepted the deed and marked her name in blood on a contract.
The woman never saw Cubare again, but she did see the man she loved and at once he greeted her affections with the same ardor. The two had a short courtship followed by a quick marriage, and soon the woman's belly began to expand with life. As the months passed, though, the life within the woman's womb seemed to siphon the life of the woman. Her mind wandered back to the deal she had made with Cubare, and that soon her life would be over and her soul the property of the devil. She could not renounce her deal even if had wanted to -- and she did not want to, as she loved her husband so much -- as the quickening in her stomach was the result of that deal. The lives of her children were the fruit of her wickedness, and she hoped she would live long enough to warn them of her folly.
It was a false hope. Her water soon broke and the birthing process began, one in which her body gave out. For she had not carried one baby inside of her but two identical twin girls with black hair, brown eyes, and skin that had a yellowish, brown tint, far closer to her complexion than his. These features she did not see in her human life as she died to give her daughters life.
But she rose again.
Her skin, once draped with pallor, now darkened and became blotchy. Her teeth sharpened and horns appeared on her head. No longer burdened with a soul, she joined the fate of all who signed Faustian contracts and died without repentance. She became a Morlock, a soulless being without conscience. In time, she would be able to feign a human appearance and entice others to lose their souls in the same manner as she had given up hers. Immediately upon her resurrection, though, evil thoughts had flooded her mind. She slew the midwife that had helped birthed her children and her husband, a man she had once loved but now felt nothing but contempt. In the confusion she had taken one of the twins...but that twin is another story entirely. This is the story of the twin left behind. That twin was the woman who would become Queen Gaia.
Now this twin was very fortunate to survive this confusion, but with her father dead, her mother nowhere to be found (and certainly in no condition to raise the child in her murderous, demonic state), the baby girl was an ill omen and not wanted by the village in which she was born. Noises were made about putting the baby out of its misery, but these whispers were dispelled by the nobleman under whose protection the village was under. Upon learning of this situation, he and his wife arranged the baby to be spirited away, and into the care of his elder sister who had long passed the age of childbearing. The baby would be raised as her adopted daughter, and though she would inherit neither land nor riches, she would be cared for as well as the lord's own children. The baby was named Gaia.
The baby grew into a child, and from there into the bloom of youth. Gaia grew up keenly aware of the differences between herself and her "cousins" but rarely envious. As the girls were groomed to be married and the boys groomed to marry, she enjoyed the freedom in not being forced to carry on a line. She instead used her position of leisure to learn and to engage in the foolishness of reading. She expected nothing more than to gather knowledge for its own benefit, and to live and die a maiden.
Fate, though, brought it so that the King of the Earth visited the land. It was known that the young King was searching for a wife to become his Queen, but no woman he had found had met his favor. Though many women were willing to do anything to become a queen, few had any desire to actually rule at his side, as most dreamed solely of being cared for in luxury. He wanted a woman who was insightful enough to manage the position as queen, even if it technically carried no real power, not to merely stand by him like a doll and bear his children.
Of course, the nobleman, Gaia's adoptive uncle, had no idea of what the king would want. He instead instructed his daughters to doll themselves up and present themselves to attract and seduce him. For the first time, Gaia was made to practice her speech and walk with the utmost scrutiny. As unlikely as it would be for the King to choose her, she had to be presented in the hopes that her difference would make the lord's natural children far more palatable. And if by some quirk, the king did pick his adopted niece, the nobleman would be the uncle to a queen.
And so the daughters of the nobleman were presented to the King, but they did not find his favor. Gaia was presented to the King and at once, he saw through the artifice and ceremony that turned her trophy instead of a person. Her hair, skin, and eyes drew him in a way he had never felt before. She had captured his heart and attention, and he had captured hers.
Despite this, it was not a quick or easy romance. The King had his duties and there was great resistance that the fair king of the Earth had fallen for a brown-eyed cow with "dark" skin. Despite this, he courted Gaia intently, summoned her to his lands, and eventually, they were married. Fear turned into acceptance and in short order, the perfectness of this match was evident. His gregariousness and exuberance balanced with her reserved and rational nature, but both of them were incisive in their observations, which prevented Earth from coming into conflict with the other. She even met with Queen Serenity at state functions, although the Queen of the Moon never seemed to be quite at ease with her.
Then Gaia became pregnant.
She worried that she would die in childbirth, as she had been told had happened to her birth mother. She worried she would make a poor mother, as while she was thoughtful and considerate, she was not naturally tender and warm. She did have a connection to Earth, one that was not inborn, but was honed through the freedom of her youth.
Prince Darien of Earth was soon born. He exited Gaia's womb on a moonless night, in the midst of an earthquake, one that Gaia knew was not merely the tremors of the planet but caused by something...alien. When was born, she felt as though part of herself had died and because of this, Gaia chose to never conceive again. The King agreed. This son would either be the hope of the world or the end of his line. Since Darien had taken after his father, both any questions about paternity and fitness of blood soon faded.
Queen Gaia lived the life of a royal wife. She had been born a commoner, but her blood had been tempered into blue steel.
And then the attack of the Negaverse on her planet ended her life. She had felt the attack before it happened, as she suffered the same ill feelings that stirred her the night her son was born. Yet her premonition could not save her. Like countless others she was never reborn in the cycle of life and death, but her spirit lived on with the earth she inherited.
Edit: Here is for the King, Darien's father. It is shorter, as he's not as interesting.
The Story of the King of the Earth
Magic ran in the blood of the royalty of the Earth, as it was from magic the royalty had sprung.
Once upon a time, the Earth was chaotic and violent. It had tried to once unite under the auspices of a human who received power that rivaled the actual gods, but when that power turned rebellion against the divine, the sun god, the moon goddess, and their father all struck back. That man was defeated, banished to the darkness and cursed with an awful legacy. He became known as Shade. He slowed his aging to a crawl, hungering for the life essence of others. Yet he was still not allowed to remain eternally youthful, and so when his vessel would grow older after centuries of use, he would force his curse on another, giving them both his memories and his hunger. For millennia this passed, and with time, Shade strengthened once more.
In that time, the actual people of the Earth had become divided. One woman, Diana, was powerful enough to combat Shade, and though she defeated him easily, she could not kill him, which led to her not being trusted. She and those who followed her left Earth to a new realm in the wandering stars of the night sky, where other humanoid beings had already lived and welcomed her wisdom. That woman was the first Queen of the Moon.
So eventually, Shade struck again. This time, there was no help from the any supernatural hero, or even the other planets in the Solar System which were much more advanced. Humanity was divided even more than ever in terms of tongues and ideology, and yet Shade, a singular being and once seen as the hero of humanity, was now the villain.
The people of the Earth were on their own. They adapted.
They devoted themselves to become warriors of the supernatural by learning magic themselves. Whereas those of the other planets focused on one attribute or element, the people of Earth used a diversity of approaches to accessing paranormal and preternatural abilities. As they developed skills, certain lines of heredity carried a greater potential for this skill. The symbol of their house, the red rose, became ubiquitous with apotropaic magic. With help from Guardians of the Dream Dimension, this line finally defeated Shade and rendered him into hiding for well over a millennium, until long after the fall of the Moon and Earth Kingdoms. However, there was a cost, as the lifespans of these individuals were shortened.
They were warrior sorcerers and in gratitude, they had the rulers of a new, unified people of Earth.
Magic ran in the blood, but it had grown thin over the years, and these deeds became legends.
The curse on the blood had not grown weaker on the rulers of the Earth and be it misfortune or malice, the royals tended to die young. Thus the royalty married young and wisely, having no time for foolish choices. It was through these circumstances that the "King Magus" Damon came to the throne, succeeding his father who had died in middle age like all his fathers before him. Despite the title, Damon had not a speck of magic of him, and he was only known as the King. As he was raised to rule at an early age, he was taught to temper his impulses and distance himself. A King was never allowed to wear his heart on his sleeve or to put his own desires over those of the realm. He was severe in his eyes, yet often silent in his mouth, more a mannequin than a man. His courageous and competitive impulses were tempered into a drive to become an even greater king than his father.
That boldness also extended to his pursuit of a queen, as he looked for an uncommon woman to rule at his side. Gaia fit his criteria, and over time he won her heart and hand in marriage. She gave birth to Darien and he began the process of training of his son to force him to know how to fight to defend. His father fostered the valor that as king he could not himself indulge, raising him to be chivalrous and active. He also trained him in the art of the rose and kindling what little magic remained. This served the royal family well when another threat - one before the Negaverse, emerging from the castoffs from the Moon Kingdom - arrived to take over the earth. The king, the prince, and the royal guard drove off this threat. This strengthened the rule of the King, and it seemed that the curse on his bloodline had been averted.
Thus when Prince Darien of Earth began to court the Princess Serena, King Damon was doubly pleased, as not only would the Kingdoms of the Earth and Moon unite, but that he dreamed that he would be able to do what no King of the Earth had done since the battle against Shade; live to see his own grandchildren.
Then the attack of the Negaverse came out of the blue, and Earth was swiftly taken over by the powers of the sorceress, Queen Beryl. Unlike the "King Magus" her powers were fast and her army was strong. The King allowed his son one last visit to the Moon, with the hopes that Queen Serenity would take him under her protection. He never saw his son again as the forces of Beryl overwhelmed the King, and he died, just as his forefathers before him.