Mariemaia Khushrenada is the daughter of Treize Khushrenada, the Duke of OZ and Leia Barton, daughter of Dekim Barton and sister of the real Trowa Barton, conceived during the meeting between the two on Colony X-18999 (as detailed in the Episode Zero manga). Supposedly dying at an early age, Mariemaia later resurfaced as the figurehead of the Mariemaia Army, which largely consisted of soldiers who had been loyal to Treize. Thanks to her grandfather Dekim's grooming, she was fully prepared to ravage the Earth and take her place as the World Sovereign, believing this to be what her father, Treize, would have wanted. If Operation Meteor went as planned, the young girl would be ruler of Earth, as stated by her uncle, though, in fact, she would be a puppet ruler as seen by her grandfather.
Mariemaia kidnapped Vice-Foreign Minister Relena Darlian so there would be no opposition to the Barton family's rule over the Earth. The occupation of Earth was quick with no resistance and victory seemed assured, that is until a number of insurgents, including the Gundam pilots fought back. Mariemaia felt that their efforts would soon be exhausted, until the populace stands up and Wing Zero destroys the shelter.
Dekim urges her to take up her rightful place as ruler of Earth but Relena slaps sense into her, literally. She then saves Darlian's life when her grandfather tried to kill her. Lady Une soon has her rushed to medical attention. Soon afterwards she makes a full recovery and is adopted by and placed in Une's custody.