Relena was originally one of King and Queen Peacecraft's two children. The Earth's Sphere Alliance thought that the Peacecraft's kingdom, the Sanc Kingdom's, peaceful idealism was getting in the way, so they made plans to eliminate them. In AC 182 (when Relena was only two years old), the Alliance launched a mobile suit attack on the Sanc Kingdom. The kingdom was destroyed. King Peacecraft and his wife, Katarina, died in the attack, and their children, Milliardo and Relena, were reported missing.
Nine years later we see Relena, daughter of Vice Foreign Minister Darlian, in Episode Zero. It seems that the Vice Foreign Minister found her after the attack on Sanc Kingdom and adopted her, as a last favor to the dead king. The manga starts Relena's story with her adoptive mother telling her to have a good day. Relena only bows slightly in acknowledgement. We see Relena sitting at her desk at her school, St. Gabriel's. Female classmates walk into the classroom, saying hello to her. Relena doesn't reply. They see a boy outside the window, and the girls talk about him being a war orphan.
Relena bumps into the boy later that day, making him drop his books, and they stare at each other for a moment. The boy then picks up his books, dusts them off, and promptly leaves without saying a word. Later that day, Relena spots him again, this time on the roof of the institute. She considers apoligizing when she hears the boy talk about taking refuge in the Darlian Estate rather than St. Gabriel's Institute. Relena is shocked.
The next day, she sees him sitting by himself under a tree. The other girls in her class make rude remarks, and Relena wonders why she feels as if the boy and her are somehow alike. That night, there is an attack on a military base nearby. Relena takes a walk in her family's garden. A rogue soldier walks by, recognizes her, and decides to take her hostage. He hears somebody coming, and drags Relena along with him as he runs. He is stopped by a mobile suit, the pilot being Zechs Merquise. Unbeknownst to Relena and her captor, he is actually Milliardo, Relena's brother who adopted a fake name under which to avenge his fallen kingdom.
The kidnapper forces Zechs out of his mobile suit, holding Relena at gunpoint. As Relena's captor makes a remark about the mobile suit, Zechs seizes the opportunity to shoot the kidnapper in the hand, making him drop his gun. Relena runs to stand by Zechs. Zechs tells him that he doesn't want to have to kill him in front of the girl, and tells him to leave; the kidnapper runs (of course).
Zechs smiles at Relena, thinking of how much she has grown since he had last seen her. Relena just stands there, looking at his gun. He remembers that he has given up his former name and taken up arms to avenge the destruction of the Sanc Kingdom. He asks if she is all right as he holsters his gun, calling her Princess. She asks if he's a knight on a terrible dragon if she's a princess. He smiles, and says that he is a prince of the stars. Somebody calls Relena's name, and Zechs excuses himself and walks away.
The next day, Relena overhears some girls at her school saying that the boy (the war orphen) had left. She goes to the roof to look for him. He's not there. She looks down at the entrance to the school and sees him. She rushes down the stairs, but when she gets there, he's gone. She introduces herself, as she feels she should have done from the beginning.
Four years later, we see Relena again at the series' start. The rest is history.