The lone daughter of an old, aristocratic family, Hildegard von Mariendorf (Japanese: ヒルデガルド・フォン・マリーンドルフ) was instrumental in garnering support for Reinhard von Lohengramm's Imperial ambitions from the more liberal members of the lower high nobility reformer nobles. Eventually, she became very close to Reinhard, and the two were married. Upon Lohengramm's death, Hilde became the regent to their son, Alexander.
Upon the death of Kaiser Friedrich IV, Hildegard von Mariendorf counseled her father, Franz von Mariendorf who at that point intended on supporting Prince Otto von Braunschweig, to instead through the Mariendorf familty's support behind the ambitious Reinhard von Lohengramm. Her father immediately consented, and Hilde then arranged a meeting with Reinhard. In exchange for promises that her family's financial holdings would remain intact, Hilde gave her support to Reinhard. Hilde tean began meeting with reformers--typically members of the lower nobility--to raise support for Reinhard. When civil war broke out, Reinhard, who had taken a liking to Hilde at their first meeting, hired her as his personal secretary and advisor. Throughout the civil war, Hilde spent most of her time solidifying Reinhard's political power on Odin and laying the groundwork for the reforms that would begin when Reinhard seized the throne.
After the end of the civil war, Reinhard von Lohengramm had the Imperial Regent, Klaus von Lichtenlade, arrested, and assumed the title of Imperial Prime Minister. He refused to declare himself Kaiser, as doing so would be killing the five-year-old Kaiser Erwin Joseph II. With most of the high-nobles killed in the civil war, Reinhard and Hilde immediately began reforming the social and political structure of the Empire. Corruption was rooted out, and the populace was given unprecedented liberty. Though Reinhard failed to deliver the democracy the reformers had hoped for, they were very pleased with his reign and did not dissent even as he dissolved the aristocracy.
When Reinhard began making preparations for an invasion of the Free Planets Alliance, Hilde advised him not to. It was, she believed, more important for him to focus on cementing his rule. Reinhard countered by saying that people needed an enemy to unite. Hilde agreed, but where Reinhard saw this enemy primarily as the Free Planets Alliance, Hilde thought the old aristocracy filled that role.
Attending a ceremony at the Imperial Miltary Cemetary on Phezzan in August 800 SC, Reinhard was attacked by an Imperial Fleet officer. Although the attempt on his life was easily prevented, this did not prevent the would-be assassin from accusing Reinhard of complicity with Braunschweig in the nuclear attack on Westenland three years before. The accusation devastated Reinhard who had allowed the attack to happen to bring an early end to the civil war . That evening, after Hilde visited him, Reinhard admitted he did not wish to be alone and asked her to stay with him.
The following morning, Hilde returned home alone but Reinhard arrived only shortly afterwards and asked her father for permission to marry her. His stated explanation was that he did not wish to follow the example of the 'lecherous' Goldenbaum Kaisers. Count von Mariendorf diplomatically advised that Hilde was emotionally distraught but did not refuse. Hilde's own reactions were denial and she declared that Reinhard did not love her and as acting out of obligation. When asked if she loved him, she claimed uncertainty.
During Oskar von Reuenthal's revolt, Hilde discovered that she was pregnant. Although at first she chose to hide this from Reinhard but shortly after Wolfgang Mittermeyer's return from Heinessen she told Reinhard, who renewed his proposal. This time she accepted and they were married early in 801 SC.
Little is known of Hilde von Mariendorf's reign as Kaiserin regent, history recorded her as the ruler who nurtured Reinhard von Lohengramm's empire out of its infancy. At his deathbed, Lohengramm advised Hilde in two respects: first, that if his son, Alexander Siegfried von Lohengramm, was unfit to rule, that he should not rule; second, that if she felt it right, Hilde should go forward with creating a constitution for the throne.