One of the central themes in Bishoujo Senshi Sailormoon is friendship. Through the love and strength her comrades gave her, Usagi was able to save the world on numerous occasions. Human beings are drawn to one another, because we are social creatures. We want interactions with other people. We want to have fun with one another, comfort each other when we're feeling down, laugh with each other on an inside joke, and share the experience of living with those we hold dear to our hearts. Chibiusa and Hotaru are no exceptions to this fact.
Chibiusa had just been through quite an ordeal. She needed to travel through time to save her mother. She was scared, alone, and needed to go through such trails that no child should have to suffer. Her emotions and dreams were twisted to create a false image of her future self. In the manga, she watched one of her only friends, Sailorpluto, die right before her eyes. In such a short time, the young girl needed to mature and save not only her mother, but the entire world. She returned briefly to the home she fought so hard to save, but soon returned to train as a real Sailor Senshi. She made some friends her age, and began to fully understand her destiny as the future leader of the world.
Enter Hotaru, a fragile, lonely girl who has no friends. She is possessed, frightened, and has no one to turn to. She resembles Chibiusa when she first arrived in the past, after her home was destroyed by the Black Moon Clan. Just as Sailormoon provided Chibiusa with the emotional and physical support she needed, Chibiusa returned the favour by becoming Hotaru's friend. For once, Hotaru had someone there for her. She didn't have to face the demons alone.
However, Hotaru wasn't the only one benefitting from this friendship. Despite the extraodinary circumstances that took place in the young girl's life, Chibiusa still had a lot of growing up to do. She knew she was loved by her family and friends, but she had to learn how to share that love with other person. She needed to think outside herself, and realize others need help just as much as she does. I think her friendship with Hotaru taught this to Chibiusa very well. She saw how much Hotaru suffered, and wanted to ease that pain. She even shared the healing powers of the Ginzuishou with Hotaru, a gem that symbolized her heart, mind, and soul. After learning Uranus, Neptune, and Pluto were going to kill Hotaru because of the dangers Sailorsaturn exposed to the world, she wouldn't have it. Much like her mother, Chibiusa wanted to find a way to save the world without offering Hotaru as a sacrifice. In the anime, even as her pure heart is being stolen from her chest, Chibiusa looks through the pain, and smiles at the image of a possessed Hotaru. Thinking her friend is alright, Chibiusa dies with a warmth in her cold body. Chibiusa is never angry in either the manga or the anime when she awakens from her death. She understands it was not Hotaru's fault, and continues to care deeply for her friend. This says a lot about the young girl, who only a season before, rejected her love ones for their seeming lack of love towards her (creating Black Lady). I realize Wiseman corrupted Chibiusa's mind at this time, but he also took advantage of her young feelings (i.e. her childish selfishness and desire for attention). By becoming friends with Hotaru, and sharing experiences with one another, I feel Chibiusa matured even more, and grew out of that child-like selfishness by a very large amount. That's not to say she didn't have her moments in the series, but compared to even a season ago, her growth as a person was greatly apparent.
During her last battle, Hotaru finds the strength inside her to fight against the being that ruled over her life for so many years. Mistress 9 chased away her friends, fought for control of her body, and made Hotaru wish she was never born. To have the courage to battle such a fiend, I admire Hotaru, and believe she wouldn't have had the willpower to without the love shown to her by her one friend, Chibiusa. She refused to let Chibiusa's future and companions be destroyed by Mistress 9. Saturn's encouragement certainly helped, but I doubt that alone would have been enough for Hotaru to truly sacrifice her own life for the sake of her friend.
Destiny surely had a hand in these two discovering one another. Even before their fate as Sailor Senshi was discovered, Hotaru and Chibiusa had a bond that could not be broken by the darkest of evils. They shared their hearts, and helped each other grow into better people. Usagi's friendship with her companions helped her save the world; Chibiusa and Hotaru's friendship helped save each other.