Endymion is the Shepherd Prince (or King) that the Greek moon goddess Selene (Luna in Roman) fell in love with. After seeing him, Selene asked Zeus to grant him immortality so she could be with him forever. Zeus granted him immortality--and eternal sleep. This allowed Selene is see him whenever she pleased, and simply had to arrive at his sleeping place to admire him up close.
Selene bore Endymion 50 daughters, also known as the Menae, which represented the 50 months of the four Olympic Years.
In some adaptions, Endymion is the king of Elis and Olympia.
In other adaptions, it is said he and Selene are the parents of Narkissus, a Greek youth.