Relena was born in April 7, AC 180, and is the daughter of Sanc Kingdom rulers, King Peacecraft and his wife, Queen Katrina, who adopted the original Heero Yuy’s “Total Pacifism” beliefs for Earth. But in AC 182, the Sanc Kingdom was attacked by the United Earth Sphere Alliance, crushing the Peacecraft monarchy. Vice Foreign Minister Darlian, a friend of the Peacecrafts, eventually adopted Relena, and she was renamed Relena Darlian.
In the year AC 191, 11-year-old Relena attends The St. Gabriel Academy. There, she would meet an unnamed boy . Later on, her home city would be under attack, and she was then held hostage by an unnamed assailant, until she was rescued by Zechs Merquise . Zechs, having abandoned the Peacecraft name, introduced himself as the “Prince of the Stars”, and walked away. The next day, the unknown boy is gone before Relena could introduce herself to him. Rumors that spread around the school said that the orphaned boy was part of the attack on a base near the town.
During a shuttle flight with Vice Foreign Minister Darlian, Relena witnesses a battle between Wing Gundam and a Leo mobile suit. She then went to the Gundam’s crash site, and was shocked to discover that the pilot is a boy who is the same age as her. Before she can know who he is, the boy fled the scene, stealing an ambulance. Back at her school, Relena discovers that the new student was the same pilot whom she met earlier, and later on, she would try to invite Heero to her birthday party, but Heero tears up the invitation, and threatens to kill her.
Vice-Foreign Minister Darlian was killed in an assassination attempt by Lady Une, and as he lays dying, he tells Relena that he is not her real father, and that her real father is King Peacecraft. She was then saved by Doctor J’s workers, and upon meeting him, she learns the truth about Heero Yuy.
Relena intervened during a rematch between Heero and Zechs at Antarctica, but Lucrezia Noin told the truth about Zechs; that he is actually her older brother, Milliardo Peacecraft. After that, before they were forced to escape, Zechs asks Noin to protect Relena.
After the fight, Relena and Noin moved to the reconstituted Sanc Kingdom. Aside from her regular duties as the ruler, Relena opened a school where she can teach her family’s beliefs. She would gain three more students: Heero, Quatre Raberba Winner, and Dorothy Catalonia, the granddaughter of Romerfeller head Duke Dermail.
Despite it being a neutral nation, the Sanc Kingdom would once again be under attack by the Romefeller Foundation, and, despite the best efforts of Noin, Quatre, Heero, and the Maganac Corps, Relena was forced to surrender to Romefeller and dissolved the country. She was then taken into Romefeller custody, and was given an offer to become the new head representative of Romefeller. Relena accepts, taking on the title as the “Queen of the World”. In her first act, Queen Relena announces the dissolution of all the national borders, and unifies the Earth as the World Nation. Suddenly, she was shocked to learn that Zechs Merquise is the new leader of the colonial rebel faction, White Fang, and that he officially declared war on the World Nation. After the declaration, former Romefeller leader Treize Khushrenada returns, and asks Relena to resign from her position; she accepts, on the condition that she goes to space to personally confront her brother.
In space, Relena would confront Zechs and try to talk some sense into him, but he would not listen to her, and was instead held captive inside the Libra, White Fang’s space station. She would be found by Heero, and again, they confronted Zechs. After that, Relena was sent to the World Nation’s asteroid base, MO-2, where she witnessed the final fight between Heero and Zechs.
At the end of the Eve Wars, Relena becomes the Vice-Foreign Minister of the new Earth-Sphere Unified Nation. On her shuttle, she sees a teddy bear and a card left by a worker. She discovers that the worker was Heero, and after tearing up his card, she smiles at him. Berg mentions that with the Romefeller Foundation being led by “that Peacecraft girl”, he will have to act independently from now on. She was the mysterious visitor who is listening to Doctor J’s story during his imprisonment.
Relena is now the Vice Foreign Minister for the Earth Sphere Unified Nation, is abducted by Barton Foundation representatives while attending a meeting at the L3 Colony, X-18999. She meets Mariemaia Khushrenada , who is planning to use Relena’s influence with the people for her plans to rule the Earth. During the fight against the Mariemaia Army in Brussels, Belgium, Relena admits to Mariemaia that she gave up the Peacecraft name, and explained that what is needed now is the heart to hope for peace, and that hatred would lead to more hatred. After the battle finally ends, Relena once again resumes her duties as the Vice Foreign Minister, starting the Terra Formation Project, which Preventer Agents Zechs and Lucrezia Noin are working on.