Queen Metallia is the true mastermind behind the Dark Kingdom, a malignant and amorphous being of pure energy rather than a creature with a fixed body. In the anime, she is portrayed as an ancient evil entity born from radioactivity within the Sun, and she was the force responsible for the destruction of the Silver Millennium, the former lunar civilization, before being sealed away by Queen Serenity and the power of the Silver Crystal. During the events of the series set in the twentieth century, she operates from her prison at the North Pole, working through the human witch Queen Beryl and the Dark Kingdom to drain energy from the Earth and to recover the Silver Crystal so that she can be fully freed.

In terms of personality, Queen Metallia is not a conversational character with a developed inner life, but rather a primordial, impersonal force of malice and darkness. She is described as malignant and malevolent, and her behavior reflects a single-minded hunger for power and destruction rather than any recognizable emotion or code. Her motivations center on absorbing energy, expanding her own being, and plunging the world into darkness. In the anime, her power is closely tied to that of the Silver Crystal, and each time one of these forces grows stronger, the other is affected in turn, creating a direct opposition between her and Sailor Moon, who channels the life-affirming power of the same crystal.

Her role in the story is that of the ultimate antagonist of the Dark Kingdom arc. She first appears as a giant shadowy entity during the ancient invasion of the Moon Kingdom, and throughout the modern storyline she remains the power behind Queen Beryl, having granted Beryl her abilities and ordered her to find the Silver Crystal. Unlike Beryl, who acts and schemes openly, Metallia remains in the background as a distant, godlike presence until the final confrontation. When she is eventually freed, she proves strong enough to resurrect herself and takes possession of the dying Queen Beryl, transforming into a towering, goddess-like creature that battles the Sailor Guardians directly before Sailor Moon destroys her using the Silver Crystal. In the English dub of the anime, she is referred to as the Negaforce.

Her key relationships are defined by this role as the supreme ruler of the Dark Kingdom. Queen Beryl is her primary servant, a human corrupted and empowered by Metallia to carry out her designs, though in the anime's climax Beryl is ultimately discarded and absorbed when Metallia revives. She also has a significant connection to the Silver Crystal and to Sailor Moon, who serves as her direct opposite and the only force capable of defeating her. In other versions of the story, including the original manga, she also uses Prince Endymion's reincarnation as a host after he is captured, but in the anime her focus remains on reclaiming her full power and overwhelming the Earth with darkness.

Queen Metallia does not undergo character development in any conventional sense. She appears, acts as a force of primordial evil, and is destroyed, without gaining new traits, changing her goals, or reflecting on her defeats. Her significance lies entirely in her power and her position as the dark heart of the first major conflict in the series.

Her notable abilities are centered on her nature as an energy being. She can absorb energy from the world around her and use it to expand her size and strength, and in her most powerful state she can spread her dark form to cover a city or even the entire planet, bringing chaos and violence to those beneath her. She can create monsters, called youma, to serve her, and she can manipulate and empower humans such as Queen Beryl. She is also capable of resurrecting herself, as seen when she returns on her own power in the final battle, and she can possess other beings, as when she takes over Beryl's body. Despite her immense power, she is ultimately vulnerable to the Silver Crystal, whose pure light directly counters her darkness and entropy.