Miscellaneous thoughts on Pluto:
We see in the manga that she’s a child during the Silver Millennium. So she grew up with the other Sailor Soldiers as her role models. Her job was to guard the time gate and that seems like a pretty important thing to keep doing when your kingdom is under attack, so it’s highly likely that she never died and was thus never reborn like the other girls. So she grew up (somehow) and watched those people she looked up to and admired, the soldiers and royalty, be born again as tiny insignificant babies. When she met them next they had no idea what the hell they were doing. I can’t imagine, really, what that would have been like for her.
Then you got Chibi-Usa. Approximately everyone has written about how she feels about Chibi-Usa, but really it’s kind of a parallel to how Pluto might have felt. Surrounded again by these amazing women and role models, feeling pretty intimidated, maybe even having that same first formative child-crush on Endymion. Then Pluto sends her back in time and you know she’s gotta be thinking “Ok, you’re going to meet them but when they were younger, and you’re never going to be able to look at your role models again once you’ve seen them fart in their sleep”.
I don’t know where I’m going with this, basically Pluto is always seen as the oldest and most mature because she’s mysterious and timeless but for a while there, she was the kid.