Chibiusa
After the accident in her father's lab, Chibiusa is the first person to reach out to Hotaru. At school, Hotaru is picked on and mocked by her classmates, all of whom think she's creepy and weird. She always wears long sleeves, looks gloomy, and is terribly weak. On top of that, her father is the "creepy professor" who was expelled from Infinity Academy. Hotaru leads a lonely existence, with only her deranged father and his conniving assistants for company.
In the anime, the two meet when Chibiusa's hat flies off into the wind and Hotaru catches it for her. In the manga, Hotaru is attacked by a daimon and Chibiusa and Usagi save her in their sailor senshi forms. From there, Chibiusa is enthralled by Hotaru. For a class project, she builds a replica of Neo-Queen Serenity's Holy Grail with the help of Mamoru and Usagi. Instructed to give it to someone special to her, Chibiusa decides to give it to Hotaru. Hotaru doesn't accept the gift, but appreciates the gesture all the same.
Later, Hotaru invites Chibiusa out. When Hotaru is late, Chibiusa stops by the research lab to see if she's still there - and sees Hotau's artificial cyborg implants. Flustered, Chibiusa flees, and Hotaru is left to agonize alone.
I didn't want anyone to know about this cold, bloodless body. I thought I would die all alone like this. I finally made a friend ... Do I need to live so badly that my body is turned into this? Is there a reason for me to live? ~ Hotaru, Act 29
Chibiusa realizes that in running away, she hurt Hotaru's feelings. Later, when she learns of the plan to kill Hotaru to prevent Sailor Saturn's coming, she rushes to find her and save her. "It doesn't matter what Hotaru looks like or what she is. It doesn't matter what her fate is. She's my friend!" A decidedly noble effort, but by that point, Mistress 9 has just about overpowered Hotaru and she is able to take the Ginzuishou from Chibiusa.
As Hotaru struggles with Mistress 9, Chibiusa is slowly dying away, barely supported by Mamoru's powers. When finally Hotaru manages to break away from Mistress 9, taking Chibiusa's Ginzuishou with her, she returns it to her friend and the two are able to talk for a minute before Hotaru disappears. Hotaru is so glad that she was able to meet Chibiusa, that the two were able to become friends.
Hotaru: This may be strange, coming from another girl, but I feel that we were destined to meet.
Chibiusa: Me too ... I thought that as well. ~ Act 33
When Sailor Saturn arrives, it's clear that she is not the same person as Hotaru. However, after Saturn is reborn again, the two are more connected and she joins Sailor Chibi Moon in the fight against the Dead Moon Circus. Afterwards, the two are nearly inseparable until Chibiusa returns to the 30th century; Hotaru is extremely saddened to see her best friend go.