Lady Une said it best when she stated that the seven-year old child prodigy was not fulfilling her father's ideals, but was just being controlled by her grandfather, Dekim Barton, the brain behind Operation Meteor. Dekim wished to continue said operation, which was originally meant to wipe out the Earth, so that everyone would move to the space colonies.
While mariemaya took Relena to her stronghold in Brussels, Dekim implemented Operation Meteor. Trowa, Duo, and Heero managed to foil his plans for the second time around, but Dekim still escaped with all of his Serpent mobile suits. He rejoined Mariemaya and Relena while the Serpents faced the Gundams, Noin, and Zechs.
While the others fought the Serpents, Heero grappled with Wufei, who had taken Mariemaya's side in the conflict. The fight soon ended, and Heero made his way into Mariemaya's stronghold. As Heero destroyed the fortress's shield and his Gundam, Lady Une protected Relena and Mariemaya from the falling debris and then told Mariemaya that what she was doing was not what Treize would've wanted.
Mariemaya, believing that she had emerged victorious, began to celebrate, but was abruptly stopped by a well-deserved slap from Relena. Relena convinced her that what she was doing was wrong, and Dekim shot Relena. Mariemaya jumped in front of Relena and was hit by the bullet, which completely paralyzed her.
Dekim was killed by some soldier, and Heero arrived. He offered to put Mariemaya out of her pain, but unfortunately, his gun was out of ammunition. Mariemaya is last seen with Lady Une, in a wheelchair, putting flowers on her father's grave.