The Earthen Prince who was heir to the planet of beautiful scenery and sounds, resided with his court in The Golden Kingdom, in Elysion. His guardians, the Shitennou, and the priest he never met, Helios, were all loyal to him. He was noble and kind, and wished to protect his beautiful Earth.
It was at this same time that Princess Serenity lived on the moon, in Silver Millenium. Save for the Mare Serenitatis (Sea of Serenity), the moon was a quiet, still landscape compared to the resonate sounds of the Earth. Princess Serenity admired both the scenery of the Earth and it handsome prince. They each tasted the forbidden fruit of loe and often met in secret, only to have Kunzite and Sailorvenus drag them back each time. They shared a deep love, albeit dangerous and forbidden, and eventually fatal.
Endymion's inner desire to protect spanned not only his planet, but his lover as well. He was killed in battle protecting his love. Serenity was so utterly devastated that she turned a sword onto herself, plunging it into her chest. Even back then, their love was so deep that separation was simply unbearable.
While much is known about Princess Serenity's mother and her court, very little comparitively is known about Endymion. He had the Shitennou, but not a word is spoken about his parents, and we never see the Golden Kingdom, except in a ruined state which Helios shows him.
Much akin to Mamoru's childhood, Prince Endymion's past is mostly clouded. This may be symbolic to equate that perhaps none of those things matter so much in comparison to protecting and loving his one and only, whether she be known as Princess Serenity, Usagi, or Neo-Queen Serenity.