Chibi Usa, and Hotaru. What keeps their bond of friendship so strong? Let's take a deeper look into what holds them together, as best friends. *This analysis focuses on the "S" arc of Bishoujo Senshi Sailormoon.
Meeting
Chibi Usa and Hotaru met in act 24, at the Infinity Research Labs. Hotaru was having a coughing spell, and Chibi Usa asked if she was alright, but was greeted with the cold reply, "I always have these fits, it'll pass in a minute. Go away. This area is off-limits."
Right after, a cat possessed by atavism attacked. Sailorchibimoon protected Hotaru, and in the process, was hurt. Hotaru used her power to heal on Chibi Usa's wound. She introduced herself as Tomoe Hotaru (pronounced Toh-moh-ay Hoh-tah-roo), and walked inside the building.
Secrets
When Chibi Usa went to return the handkerchief to Hotaru, Hotaru showed her her bedroom, that was full of lamps. She had another coughing fit, and Chibi Usa helped her using the Mystical Silver Crystal (Ginzuishou). They shared secrets about amulets, Hotaru promising to keep the secret about Sailorchibimoon, and the Ginzuishou. She also told Chibi Usa about Professor Tomoe's gruesome experiments, which Chibi Usa promised not to tell anyone about.
The Gift
In act 28, Chibi Usa made a clay replica of The Holy Grail, which Neo Queen Serenity uses for power. Her teacher told her to give it to someone important, and she decided to give it to Hotaru. This act of friendship shows the bond they already share, even though they have only met a few times.
Cyborg
In act 29, Hotaru was supposed to meet Chibi Usa and her friends at the theater to see a movie, but never showed up. When Chibi Usa went to get her, she saw Hotaru through a window being examined by her father, Professor Tomoe. This is where it is revealed that Hotaru is a cyborg, being about half made of wires. It startles Chibi Usa, and she runs away. she later felt bad that she hurt Hotaru's feelings, by doing so, and decided to go back (though evil hail stopped her, but that is a different story).
Mistress 9
When Hotaru finally turns into Mistress 9 in act 30, she steals Chibi Usa's brooch, with the Ginzuishou, causing Chibimoon to detransform, and go into a coma. While Mistress 9 was "borrowing" Hotaru's body, Hotaru's soul protected, and comforted Chibi Usa, while she was in a coma. After becoming strong enough, she gave Chibi Usa's soul back to her, along with the Ginzuishou. They decided that it was fate for the two of them to meet, and hold hands. This is the true picture of best friends. The next time Chibi Usa and Hotaru see each other is in the "Stars" arc where Usagi pulls them apart. (Thanks to Hitsuji for the correction!)