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Grunhilde Serenity, also known as Hilde, is the eighth princess of the Serenity Royal Family and the younger sister of Gruier Serenity. Like all members of her royal line, Grunhilde was born from an artificial womb aboard the legendary Golden Ghost Ship.

Grunhilde possesses blue eyes and long, light blonde hair, which she typically styles in two buns with tails for casual occasions or a neck-length ponytail for more formal settings. Her attire changes with context, ranging from the pink Hakuho Academy uniform and a black Yacht Club hoodie during school activities to a distinctive dark red military coat with gold lining when aboard royal vessels.

In terms of personality, Grunhilde is described as reasonably sociable, though noticeably more reserved and quieter than her outgoing older sister. Like Gruier, she shares an ambivalent attitude toward her royal status and does not place great importance on her position as princess. She dislikes being left out of activities and experiences jealousy when she feels excluded, particularly regarding her sister’s adventures and relationships. Grunhilde also demonstrates a pragmatic and tactical mindset, proving willing to employ underhanded or unconventional methods when the situation requires it, a trait that distinguishes her from her sister's more straightforward approach.

Grunhilde’s role in the story initially places her in opposition to her sister. During a period of political instability within the Serenity Kingdom, Grunhilde aligned herself with conservative factions, creating a rift with Gruier. While Gruier sought to destroy the Golden Ghost Ship to end the political conflicts surrounding the royal lineage, Grunhilde aimed to use the ship and its resources to settle the succession crisis in a different manner. She deployed the Serenity Defence Forces to locate the ghost ship and, upon finding it, confronted Gruier aboard the pirate ship Bentenmaru, threatening to use force if her sister did not withdraw. The conflict escalated to a boarding action, during which Grunhilde was subdued by sleeping gas administered by Marika Kato, the captain of the Bentenmaru. Upon awakening and learning that Marika had secretly arranged a peaceful resolution, Grunhilde reconciled with Gruier, and together the sisters announced the ghost ship’s return to Serenity.

Motivated partly by jealousy over the positive experiences Gruier had while away from home, Grunhilde subsequently transferred to Hakuho Academy and joined the school’s space yacht club at the beginning of Marika’s second year. Her technical abilities grew through her yacht club training. She gained proficiency in piloting dinghies using simulation training and demonstrated significant tactical skill during the club’s internal competition for the Nebula Cup, securing second place by using a tow-rope on a competitor’s board—a placement she repeated in the actual Nebula Cup tournament. Beyond dinghy operation, Grunhilde acquired practical experience managing various ship systems aboard the luxury yacht Odette II and the pirate ship Bentenmaru. On one occasion, after stowing away on the Odette II with her sister, she assisted in executing an emergency faster-than-light jump aboard the Bentenmaru to evade detection by military vessels.

Grunhilde’s relationships evolve significantly throughout the story. Her initial rivalry with Gruier transforms into a mutually supportive bond, as seen in their collaborative efforts during various operations and their shared experiences at Hakuho Academy. Her perception of Marika Kato also changes dramatically. Initially distrustful of Marika due to her association with piracy and protective of her sister’s involvement with the Bentenmaru captain, Grunhilde quickly revises her opinion after observing Marika’s character firsthand. She eventually comes to understand Gruier’s admiration for Marika and develops her own rapport with the pirate captain, though she sometimes displays hints of envy when she feels Gruier monopolizes Marika’s attention.

During the events of Abyss of Hyperspace, Grunhilde encounters Kanata Mugen, a guest at the Serenity residence. She exhibits noticeable interest in him, frequently staring at him during meals and averting her gaze when noticed. She covers him with a blanket while he sleeps, assures him of his safety, and researches his father’s background. This behavior leads to observations from Gruier and other yacht club members that Grunhilde may have developed a romantic interest in Kanata.
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