I find a way!

Luce has been an 18 year old girl for a long time now. She will never change because she is one of the daughters of Zeus. Zeus gave her the gift of youthfulness or you can say immortality. She won't look older than 18. It is actually more of a curse than a precious gift. Her mother is a human, so she shall always get older. Her mom saw this as a wonder. Luce will see her mother and friends see die. She would be with nobody because she never gets older.

But one day she met the daughter of Hades, her cousin Melanie. She was pretty like all other children of the gods. But Melanie changed Luce' world forever. She had often had "one night stands" and guys she played for a while and that wasn't the big problem. The problem was that she killed them after she gets dull.
After she met her new toy everything changed. She met him before and he still has those beautiful eyes she saw before when she left him that night.

There were only two problems: he is mortal so he gets always older. The order problem is: he is the toy of Melanie.