Gruier Serenity, seventh princess of the Serenity Royal Family in the Serenity Star System, possesses striking blue eyes and long blonde hair often arranged in twin buns at the back. During formal events, she dons an elaborate white gown adorned with frills, a prominent front ribbon, a tiara, a necklace, and two white roses woven into her hair. At Hakuho Academy, she switches to a pink middle school uniform paired with a blue skirt.

At thirteen, Gruier embarks on a daring mission to locate the Golden Ghost Ship, a first-generation colony transport ship pivotal to her kingdom’s history. Her objective is to destroy its Artificial Womb—a facility engineered to genetically produce royal heirs—believing the monarchy’s dissolution and a shift to republicanism would restore Serenity’s prosperity. This ambition sparks conflict with her younger sister Grunhilde, who seeks to preserve the womb. The sisters’ clash culminates in Marika Kato mediating their reconciliation, sparing the newest heir birthed from the facility.

Following this resolution, Gruier enrolls at Hakuho Academy, joining the Space Yacht Club. Despite her royal lineage, she forges a bond with Marika, frequently aiding her during crises—a dynamic that unsettles Grunhilde.

Known for her approachable, sociable nature, Gruier deliberately avoids leveraging her status in personal interactions. A diplomatically trained painter honed by royal tradition, she also harbors a unique skill: the ability to discern truthfulness in others, cultivated through her upbringing.