As a child, Endymion was raised in preparation to take the throne. He was trained in swordsmanship, archery, hand-to-hand combat and politics. As the only child of the Terras, there is no Sailor Earth in his family. He was extremely close with his grandfather, Matthew, though his relationship with his father remained strained throughout his childhood.
In order to keep senshi powers in the family, Endymion was always expected to marry Diônê. The two grew to be close friends and even in a relationship for a short amount of time, but Endymion fell out of love with her when he met Serenity Lunaria. Diônê, scorned, turned to her mother and Queen Metallia to get Endymion back, only to be infected by darkness.
Endymion met Serenity when he went to her birthday party and fell in love with her on the spot. The two continued to meet in secret after that, marrying when Endymion was twenty-two and Serenity was fifteen. No one in Endymion's family knew about their relationship, but the Shittennou found out when Endymion was twenty-three. However, they only thought the relationship was just beginning.
Endymion comes from the Greek myth of Selene and Endymion. Endymion, a mortal, fell in love with Selene, a goddess. He bore her fifty daughters, but when he started to age, Selene asked Zeus to let him stay young forever. In turn, he was put to eternal sleep, never to age.
Each planet is named after the ruling family, thus making Earth only being called Earth or Terra for three generations, Endymion being the most recent.