Setsuna Meiou is rarely seen around Tokyo. She is a mysterious and calm young lady, usually working at a laboratory outside the city. The reason you never see her is that Setsuna's duty is to guard the Gate of Time as the mysterious senshi, Sailor Pluto!
Setsuna Meiou is a very mysterious person. She leads a solitary life as the Guardian of the Gate of Time, and is somewhat of a loner. When she does appear in Tokyo to help the other senshi, she is most often seen alone. This may be why she bonds with Hotaru in the last season. Setsuna is also a very logical and rational person, but tends to be quite blunt when conveying information. This may make her seem cold and insensitive, but she is really just trying to help. Since Setsuna is the eldest of the senshi, she often acts as their guide. She is very affectionate of Chibi-Usa, and acts as her guardian and best friend much of the time, especially in the "Sailormoon R" season. Because of this affection, Setsuna often acts as a link between the Inner and Outer Senshi.
Setsuna's responsibilities as the Guardian of Time leaves little room for a love life, however in the manga, it is made clear that Sailor Pluto is secretly in love with King Endymion. Although she would never say it to his face, for she is fiercely loyal to the Neo-Queen and King, she is deeply in love with him and is content with admiring him from afar.
Sailor Pluto is the first Outer Senshi to be revealed. In the "Sailormoon R"season, she communicated with Chib-Usa through the Luna Sphere, guiding her through her trip in the past. At first, we don't know who she talks to, but it is revealed that Sailor Pluto, the guardian of the Gate of Time, is the mysterious voice that Chibi-Usa calls "Puu" (Or Luna-P in the English dub). Sailor Pluto then allows her new allies, the Inner Senshi, to travel to the future and help save Crystal Tokyo. In the manga, Sailor Pluto is killed when she uses the Dark Dome Attack to stop time to prevent Prince Diamond from combining the powers of the Silver Crystals of the past and future, which would destroy the world. Luckily, in the next season, Neo-Queen Serenity brings her back to life so she can help fight against this greater evil. In the anime, this doesn't happen.
During the "Sailormoon S" series, Sailor Pluto returns to unite her talisman, the Garnet Orb which rests on top of her staff, with Uranus's Space Sword and Neptune's Deep Aqua Mirror, so the Sacred Cup can be formed. In the anime, Sailor Pluto is killed when she uses the Dark Dome Attack to stop time in order to save Uranus and Neptune from an exploading helecopter. She saved them so they could continue on with their mission and destroy the Messiah of Silence. This does not happen in the manga (To my knowledge). However, she is later revived by Neo-Queen Serenity when Galaxia threatens Earth.
In the "Sailormoon Stars" season, Setsuna returns to take baby Hotaru away from her father so she can be raised by Michiru, Haruka, and herself. Suddenly, Hotaru starts growing at a rapid rate until one day when she returns to the aga she had been during the "Sailormoon S" series. At this point, she provides the other Outer Senshi, including Sailor Pluto, the power to become Super Senshi. Sailor Pluto then joins the Inner Senshi against Queen Nehelenia and Galaxia, eventually being killed by an evil Sailor Uranus when she took her starseed. Luckily, Setsuna was reborn when Galaxia was freed from Chaos's control, and lived on in peace.
During the Silver Milleniem, like all the other senshi, Setsuna was the princess of one of the planets of the solar system. Obviously, she was the Princess of Pluto. However, because of her eternal position of Guardian of Time, she probably never appeared in the Moon Court. In the manga, there was a door deep within the Moon Castle that led to the Gate of Time where she stood guard, however it was forbidden to pass through it. If anyone did, Sailor Pluto would kill them unless they were heirs or rulers of the Silver Millenium. Even though this is not stated in the anime, it is pretty likely that it is the same as in the manga. During the attack of Queen Beryl's Dark Kingdom, Sailor Pluto was the only senshi who didn't die. Since she lives in the fourth demension, outside of the flow of time, she still looks 17, though she is at least 1,000 years old.
In the future during the reign of Neo-Queen Serenity and King Endymion, Sailor Pluto is still acting as the Guardian of Time. Like in the Moon Castle, there is a door leading from the Crystal Palace to the Gate of Time which she guards. This is often a refuge for Chibi-Usa when people are picking on her, for she and Pluto share a special bond of friendship. Since she lets Chibi-Usa go to the past in the first place to find Sailor Moon, Sailor Pluto plays a key role in the "Sailormoon R" season.