When I was writing about Makoto, I used her own personal name in the header. We first meet her as a normal teenage girl, and despite becoming Sailor Jupiter and defending justice and the like, the manga follows her life as a normal teenage girl. When we first meet Sailor Pluto, though, in the middle of the second arc of the manga, we see her as the senshi first and foremost. We don’t even realize she has a civilian identity until the third arc, when we finally “meet” Setsuna Meiou. This fits rather well, as Pluto’s arc is time in reverse compared to the others – she’s the no-nonsense soldier of the future, then upon her return she’s allowed to live a (somewhat) carefree life.
We know next to nothing of Sailor Pluto’s actual past. One flashback in the manga shows Queen Serenity telling Pluto the three rules of time that she is never allowed to break. Leaving aside the fact that she has broken all three of them by the end of the manga, what’s striking is that she’s telling this to a Sailor Pluto who looks to be about 7 years old. I mentioned before that these Sailor Senshi are not going to grow up and get married, but grow up and take on their duties as defenders of the Earth. Pluto, on the other hand, seems to have been born to this duty. It’s a rather jarring. What’s more, throughout this first arc we see that Pluto’s life in Crystal Tokyo is a solitary one… her only friends seem to be Endymion and Chibi-Usa. One would imagine even in Crystal Tokyo the other inner senshi all still hang out. Is Pluto’s task really that important?
Speaking of which, why isn’t Pluto’s task actually being the Senshi of Death? There are rumors that Naoko Takeuchi got Pluto and Saturn confused when she was writing the manga, and then gave Hotaru powers equivalent to “death” when she realized it was too late to go back and change it. In any case, guardians of time are not new in the land of fiction, or even manga, and it’s actually more of a surprise that Pluto doesn’t abuse her powers *more*, given the obvious temptation. Then again, Pluto is… well, not exactly a stoic, but her personality definitely tends towards the cooler end of the spectrum. Even when she’s living as Setsuna in present-day Tokyo, we learn very little about her personal life, her likes and dislikes, etc. She has no romantic scenes or pairings, even with the other senshi; the manga hinted she might have a crush on future King Endymion, but even that was mild. It does seem that she’s enjoying her time as Setsuna, at least. Probably the first time she’s had to relax in thousands of years.
We have no real idea how old Pluto is. Her parents are unknown, and she seems to be a young girl in the Silver Millennium when talking with Serenity. Her future is… confusing, given that she dies in Crystal Tokyo and then is reborn into the past to live in the present with Usagi and company as a (presumed) 18-year-old college student. Is she caught in a time loop? I’d like to say she isn’t, given I think that might be a bit too cruel. I imagine the combination of her own powers and Neo-Queen Serenity’s was able to work something out. (In the anime, of course, she doesn’t die in the first place, rendering all of this moot.)
Fan opinion on Pluto is mixed. As I said, there’s a lot of “with great power comes great responsibility” to her senshi powers, and that, combined with her cool and somewhat aloof personality, means that there’s a lot of fanfics and fan opinion portraying her as a manipulator. Heck, I’ve done it myself. In terms of the canon, though, I appreciate Pluto for her role as the Senshi of time, and her ability to show us what duty and sacrifice really are; but I also appreciate just as much her time as Setsuna from the third arc onwards, if only as we do see a teasing and fun side to her, and she’s a devoted parent (along with Haruka and Michiru) to Hotaru. Setsuna is time reversed from the other senshi – we see her as a soldier, then we gradually see her civilian self. As for which is her “true” self, well, that’s a hard question to ask for any of the senshi.
Setsuna for me has been a senshi to adore.. also an enigmatic one. I can definitely say that tears were shed when the world.. announced that Pluto wasn’t a planet anymore, but one of Neptune’s moons. Still the point that you mentioned of Setsuna in fanfiction.. it is quite true that she’s a manipulator.. but somewhere I’ve always read of Setsuna being the third person in Endymion’s marriage with the Queen.