Here is the scoop, Serena and Darien have been an item for centuries! In the beginning, Serena was Princesses Serena. She was in love with Prince Darien of Earth. Their like of peace was not to be, however, because of the evil Queen Beryl. She tried to take over the Moon Kingdom. She saw Darien, and wanted to have him for herself. When she was trying to capture him, Serena wouldn't let him go, and they both ended up dying. Serena's mother, Queen Serenity, used the Empierium Silver Crystal and preserved their future, but it made her to weak, and she dyed saving her kingdom.
They were re-born on earth as Serena and Darien. At first they thought each the other was a jerk, but Serena found out who he was, and really fell head-over-heals for him. He eventually grew to like her, and they became a couple again. Yet, their peace was shattered, when Queen Beryl captured Darien, and made him her right hand man. Serena came to the rescue and used their star locket to take the spell off him and set him free. In the battle against Queen Beryl Darien lost his memory, and started to dispise Serena. In the Doom Tree Series, Serena was still chasing after him. At the end of the Doom Tree Series, Darien gained his memory back, when the Moon White Night went back into Darien. They got back together.
After that Darien started to get dreams about something bad happening to Serena, and he broke up with her to protect her from whatever it was threatening her well being. He couldn't tell Serena what was going on, because he, himself, didn't know what it was. In the future Serena is known as Neo Queen Serenity, and Darien is know as King Endymion. They find out that Rini is their daughter from the future, and that the reason they were made to stay appart was for the benefit of the future. It was to test their love, so that when the time came, they would be able to with stand any obstacles.