Why are the themes of Sailor Saturn and Sailor Pluto switched from their mythological counterparts?
Did Naoko Takeuchi make a mistake when naming Sailor Saturn and Sailor Pluto since Saturn is the god of time and Pluto is the god of death? I think Naoko Takeuchi knew what she was doing and I will explain.
First of all, it's important to point out that though Greek and Roman mythology is an inspiration to much of her work in "Sailor Moon", Naoko Takeuchi did not confine her characters to their mythological counterparts. For instance, Mercury was the messenger for the gods and is usually associated with speed and commerce, which has nothing to do with Sailor Mercury, who is known for her ice-based attacks and intelligence. Also, Vulcan was the god of fire, though Sailor Mars is most associated with fire. So Naoko Takuechi never really stuck strictly to Roman and Greek mythology.
Sailor Saturn is the Soldier of Silence or the Soldier of Destruction. It's important to point out she was never called the Soldier of Death or the Soldier of Death and Rebirth as many fans seem think. She only has the power to destroy and cannot restore or give rebirth to anything. In the manga, Sailor Saturn told Super Sailor Moon that it was Super Sailor Moon's job to bring everything back. Sailor Saturn also wields the Silence Glaive, which is also called the Silence Scythe in Japanese written language.
The Roman god Saturn is the god of agriculture. He is usually depicted holding a scythe to harvest the land. He usually represented the harvest and its destruction to the Romans. Saturn had the power to bring a good harvest or destroy the harvest so farmers established a festival in his honor in hopes that Saturn would bless their harvests. Sailor Saturn is also a scythe-wielding person who can bring about destruction.
The Greek equivalent to Saturn is Cronos. However, the Stoic philosophers in the Hellenistic period confused Cronos, god of agriculture, with Chronos, god of time and father of Zeus. Chronos and Cronos were two very different gods and unfortunately, this mistake has caused many references and many people to equate Saturn with Chronos, the god of time. Saturn is not the god of time.
Astrology also associates the planet Saturn with destruction and threat. It is called the "forbidden star" and is equated with terrible events and circumstances. For hundreds of years, Saturn was known as the "bad" planet. This depiction of the planet Saturn also fits well with how everyone saw Sailor Saturn in "Sailor Moon S". She was known as the "bad" Sailor Senshi, whose awakening would only lead to destruction.
Hotaru Tomoe means "Earth firefly" in Japanese. The word for Earth is better translated into "soil", which is plays into the ideas of harvest and farming associated with Saturn, the god of agriculture.
Sailor Pluto is usually referred to in the anime as the Soldier of Revolution. Naoko Takeuchi called her the "Guardian of the Underworld between times" in her Materials Collection art book.
The Roman god Pluto was the ruler of the underworld. His Greek equivalent was Hades, whose name means "the unseen one". The underworld was known to be a realm of darkness, hidden from everyone.
In the manga, Sailor Pluto said she is the daughter of Chronos, the god of time. "She has the blood of the god of time, Chronos, in her veins. The solitary guardian of the gates of the underworld..." King Endymion said about her in Volume 5 of the manga. She inherited her father's power of time and space and guards the Door of Time, which exists between times. The Door of Time is Sailor Pluto's underworld, hidden from everyone. Sailor Pluto's non-time powers are also associated with darkness and the underworld. The wind that surrounds Sailor Pluto when she uses her "Dead Scream" attack in the anime is called the "Wind of the Underworld" in the "Sailor Moon SuperS Movie Memorial Album". It relates to both death and the underworld. Also, her attack "Dark Dome Close" refers to darkness. When using time-related powers, Sailor Pluto always calls out the name of her father, Chronos, god of time.
In astrology, the planet Pluto is associated with dramatic and drastic changes that will affect entire populations. This notion fits in with Sailor Pluto's theme as the Soldier of Revolution.
Setsuna Meiou means "Momentary dark king". Dark king definitely plays into Pluto as lord of the underworld and momentary seems like a subtle reference to Sailor Pluto's time powers.
Naoko Takeuchi didn't make a mistake. Her reasons of choosing these names for Hotaru and Setsuna make perfect sense. It would make less sense for Hotaru to be the guardian of the underworld and Setsuna to be able to destroy the universe, wouldn't it? =)