Setsuna Meiou is the civilian identity of Sailor Pluto, a Sailor Guardian who makes her first appearance in the second story arc of the series and takes on a more prominent role in Sailor Moon S. She is unique among the other Guardians because of her sacred duty as the solitary keeper of the Space-Time Door, a post she has held for millennia, even before the fall of the Silver Millennium. This responsibility has defined much of her existence, leaving her isolated from the other Guardians for a very long time. In Sailor Moon S, she is permitted by Neo-Queen Serenity to leave her post and join the present-day battle, reinventing herself as a tall, elegant, and sophisticated university student with long, dark green hair and purple eyes. Her age places her in her late teens or early twenties, making her the eldest of the active Sailor Guardians. She has a calm and analytical personality, often appearing distant, reserved, and formal, which sets her apart from the more energetic inner guardians. Her demeanor can be perceived as cold, but this is a result of her profound loneliness and the immense weight of her responsibilities rather than any genuine lack of feeling.

Unlike the other Outer Guardians, Sailor Pluto is generally more sympathetic and cooperative with Sailor Moon and her inner team. While Haruka and Michiru are often focused on their own secretive and ruthless methods, Pluto serves as a bridge between the two groups, offering advice, information, and direct assistance to the Sailor Guardians. Her primary motivation is to protect the flow of time and the future, particularly the safety of the future Crystal Tokyo and its royal family. She has a deep, almost maternal affection for the small lady, Chibiusa, whom she has watched over from her post at the Space-Time Door. Chibiusa affectionately calls her Puu, and Pluto is fiercely protective of her, acting as a guardian and mentor. This personal connection to Chibiusa provides a strong motivation for her actions in the present timeline.

During the events of Sailor Moon S, Setsuna joins forces with Sailors Uranus and Neptune, Haruka Tenou and Michiru Kaiou, to investigate the mysterious Heart Snatchers and the Infinity Academy. Although she operates alongside them, her loyalty is not as rigidly exclusive. She later becomes a central figure in an adoptive family unit with Haruka and Michiru, and after the conclusion of the battle against the Death Busters, they take on the responsibility of caring for the reborn infant Hotaru Tomoe, with Setsuna acting as a mother figure to the child. A significant part of her character development in this season involves her transition from a solitary, eternal guardian to an active participant in the world, forming deep, familial bonds with those around her.

In her civilian life, Setsuna is a university student who studies physics, and she harbors a secret dream of becoming a fashion designer. She is skilled at sewing and has an appreciation for elegant clothing. She has a few notable dislikes, including a fear of cockroaches and a distaste for eggplant.

As Sailor Pluto, her power is intrinsically linked to time, space, and the underworld. She wields the Garnet Rod, a large key-shaped staff topped with a garnet orb, which is one of the three Talismans. This item is required to focus her abilities. Her primary attack is Dead Scream, a powerful offensive technique that unleashes a blast of energy. As the Guardian of Time, she also possesses the forbidden ability to completely stop time. This is the most severe of three taboos she must obey, the others being that she cannot travel through time or abandon her post. Using the power to stop time comes at the ultimate price, costing her life. It is a power she is forced to use during the climax of the S season to save Sailors Uranus and Neptune from a fatal explosion, leading to her apparent death.