The Puu Factor
Chibiusa/Chibimoon can be linked with a number of different soldiers in Sailormoon. However, one soldier holds a very special place in her heart: Sailorpluto. Pluto befriended Chibiusa long before she knew of Usagi, Mamoru, and the others. She was a lonely, little princess, who was constantly made fun of for lacking in the sailor soldier powers her mother once possessed, and being stuck at a young age, despite her body being over nine hundred years old (in the manga at least). Her parents were often busy with business matters, the other senshi were involved with protecting their queen, and the children rejected the pink-haired princess. Her only friend was a floating toy, shaped in the image of the queen's feline advisor. Yet, a child cannot survive on the friendship of a toy alone. A child needs to interact with children their own age, and learn how to be a functioning member of society.
Thankfully, Chibiusa finds friendship in the older senshi (whom she affectionately refers to as "Puu"). She gains not only a friend, but a mentor. Someone who can give her advice, teach her tricks with Luna P, and give her the companionship both of them needed. In the later part of the R season (manga), Chibiusa becomes brainwashed, and is transformed into Black Lady. Sailormoon, King Endymion, Venus, etc. are all shocked and horrorfied by this discovery. While the soldiers try to save the lost, little girl, Pluto can sadly only watch from afar. She remembers the three taboos given to her by Queen Selenity, one of which, she may never leave her post at the Time Gates. Diana is sweet enough to volunteer herself to stand guard, while Pluto gains the once-in-a-lifetime chance to fight alongside the other senshi. Unfortunately, this leads to her breaking the third taboo: never stop time. Pluto must sacrifice her life, in order to save the world from destruction. In her dying breath, she pleads for the king to save Chibiusa, her dear friend. The soldier's death is enough to remind Chibiusa of who she really is, and finally awaken her soldier powers. Yet, her being a sailor senshi means nothing, if Pluto isn't there to congratulate her and be proud of her. Their parting is very heart-breaking, and shows just how close the two had become.
Locked inside a womb-like bubble, Saturn was never to awaken. Only a few soldiers even knew of her existence. Sounds familiar? Not only Saturn, but Pluto was also a rumoured senshi. Although not contained like Saturn, Pluto was still confined to one spot for all of time. She must have been extremely lonely, but she would face anything to keep the stream of time flowing properly. Likewise, although her life consisted of nothing but a semi-conscious state, Saturn would endure the lack of a normal life if it meant the Moon Kingdom would be safe. There is no official information on the interaction of Pluto and Saturn during the Silver Millennium, so one can only speculate how close the two were. However, we are given some hint during the S manga. At the very end, when Saturn is sending the creature known as Pharaoh 90 back to its own dimension, the soldier of death calls out to Pluto to close the Time Gates. This would trap Saturn, and Pluto is reluctant to perform the task. Yet, Saturn knowingly sacrifices her life, and leaves the Earth with a smile on her face, as the doors close behind her. Pluto cries out to the soldier, and is obviously very torn up by the affair. It can be suggested that even though Pluto and Saturn weren't close, they still held great respect for each other, and knew how similar their personal sacrifices for the princess and her soldiers were. After Saturn is reborn, Haruka, Michiru, and Setsuna make a promise to raise the baby. Their parental roles are rewarded with the awakening of Sailorsaturn, who gives them new transformation devices so they fight alongside their princess. In the Stars season, Setsuna volunteers as a nurse at the school Chibiusa and Hotaru attend. This way, she may be close to two of the most important people in her life, and watch over them as she has done countless times before.
The relationships may be different, but it's fairly obvious to see how Pluto has affected the lives of Chibiusa and Hotaru. She serves as a mentor, a kindred-spirit, and a dedicated friend.