Millennium Feria Nocturne, almost always called Millie, is one of the two heroines of Lost Universe. Her full birth name is Millennium Feria Stargazer, but she later adopted the surname Nocturne after distancing herself from her grandfather, Albert von Stargazer, the head of the criminal organization known as the Nightmare Syndicate. She had never met her family before the events of the story and was unaware of her true lineage until late in the series, when a clone of Stargazer summoned her for a confrontation. Millie originally worked as a female detective for a detective agency, but she was fired after becoming entangled in an incident involving Kain Blueriver during a mission connected to the Dugall Connection crime syndicate. She subsequently joined Kain on his space travels as an assistant and occasional private detective.

Millie is a happy-go-lucky young woman with a cheerful and outgoing demeanor. She is defined by her repeated catchphrase and driving ambition to be the best in the universe at whatever she sets her mind to. This goal reflects a confident and determined personality, though she often falls short of her lofty aspirations. Her enthusiasm and energy can sometimes come across as naive or bumbling, and she has a tendency to be somewhat reckless. Despite her cheerful exterior, she harbors a sense of determination and resilience, especially when it comes to opposing the criminal syndicate that her grandfather leads. She also displays a degree of jealousy toward Canal, the artificial intelligence of Kain's lost ship, which hints at a need for validation and attention within her found family.

Her primary motivation is the pursuit of excellence, whether in her detective work, her marksmanship, or her cooking. Beyond personal ambition, she is driven by a desire to do good and to actively fight against the Nightmare Syndicate, even after learning that its leader is her own grandfather. This decision to turn against her family and change her surname underscores her moral conviction and independence.

In the story, Millie serves as Kain Blueriver's sidekick and companion, accompanying him aboard his lost ship and assisting with various missions and trouble-shooting jobs. She functions as a supporting character who provides both combat capability and comic relief. Her role evolves as the narrative progresses, especially when her hidden lineage is revealed, forcing her to confront her past and choose her allegiance.

Key relationships include her partnership with Kain Bluer River, for whom she develops a deep loyalty. She shares a sometimes tense but ultimately cooperative dynamic with Canal, the ship's computer, whom she views as a rival for Kain's attention. The most significant relationship is with her grandfather, Albert von Stargazer, whom she actively opposes despite their blood connection. After Kain and Canal disappear at the conclusion of the series, Millie is shown living in the home that Kain and his grandmother once occupied, waiting for their return.

Her character development centers on the discovery of her true family background and her conscious decision to reject the legacy of the Nightmare Syndicate. She grows from a somewhat scatterbrained aspiring detective into a more grounded individual who accepts her past while choosing her own path.

Millie possesses several notable abilities. She is an expert sharpshooter and a highly proficient gunner, demonstrating considerable skill in combat situations. She is also an excellent cook, though her culinary efforts almost always result in explosions that destroy the kitchen she uses. Her detective skills are basic, such as using a hairpin to open a locked door, and she is generally described as an amateur in that field. Her greatest strengths are her marksmanship and her unshakeable optimism, both of which make her a valuable ally despite her occasional incompetence in other areas.