The bonds which hold people together serve as one of the many important themes in Sailormoon. Through love and friendship, evil can be conquered for the good of mankind. Hotaru and Chibiusa are a unique pair, because despite how different they may appear, the two girls are more alike than most of the senshi.
Before the guardian senshi of Selenity were awakened, each of the girls seem lonely (this is especially shown in the R movie). Minako was fighting evil by herself, Ami was out-casted for being so intelligent, Rei was regarded as a strange priestess, and Makoto frightened her classmates because of her height and impressive physical strength. Not until Usagi befriends these girls do they experience friendships that will last a lifetime and help them grow to their full potential.
Chibiusa becomes friends with her classmates, but rarely do these relationships build on her character (with the exception of the R season anime, when Chibiusa uses her power to protect Momoko). When Chibiusa meets Hotaru, her world is practically turned upside down. I'm sure she never expected a chance meeting to blossom into one of the most important events of her life. Hotaru is a weak girl, but it's not revealed until later in the S season that not only is she possessed by the evil entity, Mistress 9, but also houses the soldier of death and destruction, Sailorsaturn. The pink-haired girl is told point blank by the soldiers of the Outer Solar System that her friend must be killed, so the world will not be destroyed. This is Earth shattering news for any person, let alone a child, who only recently helped save the future of Crystal Tokyo. She has a decision to make, either allow Hotaru to be killed for the greater good, or protect her dear friend. Chibiusa picks the latter, and refuses to let Uranus, Neptune, and Pluto take the life of her new friend. This cost Chibiusa her life, but she does not blame Hotaru. She knows Hotaru would protect her in return (which Hotaru does indeed do, more so in the manga). Hotaru has to painfully watch as the life of her best friend is taken by the being that ruled over most of her life. She feels helpless and useless. However, Hotaru returns Chibiusa's protection with protection of her own, as she gathers up the strength, and manages to escape with the souls of Chibiusa and the guardian senshi. The meeting between them when Chibiusa is given back her crystal in the manga is very powerful and bittersweet. Hotaru can only say good-bye, and Chibiusa can do nothing as her friend slowly disappears before her eyes (this role reversal shows how Chibiusa becomes the helpless one). Chibiusa is crushed, and weeps for the passing of Hotaru. Although she has experienced a great loss, Chibiusa pulls herself together, and promises her future father that she will fight with the life Hotaru fought so hard to preserve for her (this echoes Hotaru's decision to fight for the life of Chibiusa from Mistress 9). This moment may seem small in comparison to other moments in the series, but it serves as a major point in two characters' lives, and shows how through friendship, people can find the strength inside themselves to push on, even in the most distressing times.
Sailorchibimoon and Sailorsaturn stand for opposite sides of the spectrum in terms of power. Chibimoon's power relates more light and life, while Saturn's power is focused on the dark and death. However, their powers are linked. Nehelenia is quoted in the SuperS manga as saying, "The darkness calls to the light, and the light shall call to the darkness again!!" This line proves to be true, because the immaculate, pure glow of the Ginzuishou forever attracts the darkness at the edges of the solar system. Since Chibimoon possess the Silver Crystal of the future, she, like her mother before her, will always draw evil towards her. Saturn has the ability to destroy planets, but her power is two-fold. Yes, there is death and destruction, but, in Saturn's own words, "With death, comes hope and rebirth." Saturn is reborn whenever her true powers are unleashed. But this is not solely linked with her physical rebirth. In the S season, there are two Messiahs. One of those Messiahs is Sailorsaturn, while the other is Sailormoon. Saturn brings the destruction, but it is Sailormoon who brings about the rebirth. Life runs in cycles of life and death, and this cycle is illustrated by the powers of Sailormoon and Saturn. When Usagi assumes her role as queen of Crystal Tokyo, Chibimoon will become the new Sailormoon. So, the young senshi will forever be linked to her best friend, because life is not eternal, but neither is death.
The friendships between the soldiers of the Milky Way Galaxy are all strong and unique, but the bond shared between Chibimoon and Saturn is especially different. Their closeness in age, their linked powers, and their ability to help each other grow as people are all traits which aid in the close bond between these two. And it's close relationships like these that make up a central backbone in the Bishoujo Senshi Sailormoon series.