Chibi Chibi

Chibi Chibi is a cute little girl who only says "Chibi chibi?!" or imitate people. 

She first appeared in episode 182, falling from the sky surrounded by feathers, holding onto a white umbrella. When she landed, her umbrella was blown away and Usagi returned it to her. 
Later on, Luna noticed that the little girl had been following them. 
Usagi asked her where she lived, where's her mum and so on, but the little girl keeps saying, "Chibichibi..". Usagi took her eyes off the girl for an instant, and the girl had disappeared.
The next moment they arrive home, Chibi Chibi is standing in the doorway of their house. Usagi's mother scolded Usagi for going out without her little sister, Chibi Chibi. Usagi and Luna were shocked, as she must have somehow hypnotized Tsukino Ikuko into believing Chibi Chibi was Usagi's little sister.

 Chibi Chibi resembles a lot like Usagi, so the girls thought that she might be Usagi's second future daughter. Setsuna confirmed that Neo-Queen Serenity only had one daughter who was Small Lady. She advised them to be careful of her, but despite what was said, Usagi really liked Chibi Chibi because of  her innocent nature.

During a battle in episode 187, Chibi Chibi touched Eternal Sailor Moon's eternal tier, and a beam of light came out and went up into the sky.
In a dreamy scene, Chibi Chibi (in a Sailor Senshi's suit) floated towards Eternal Sailor Moon. Their  hands touched.

Eternal Sailor Moon: A new power. A gentle undulation. Chibi Chibi, what are you?

Sailor Moon's tier flashed and a new, longer stick grew out of it. Chibi Chibi had given Eternal Sailor Moon her new powers. With that, Eternal Sailor Moon used her attack Silver Moon Crystal Power Kiss against the enemy.

In episode 199, it was finally revealed that Chibi Chibi was the Light of Hope. She unleashed her powers to save Eternal Sailor Moon.

Galaxia: "This glitter.. it can't be. The only light that can compete against me. The light that I released to the edge of the galaxy.

Sailor Moon's star seed went back to her, although she was still on the ground. Chibi Chibi was floating in the air, over a puddle of light. There were rings of light coming out from her.

Galaxia: The light of hope awakened?

Suddenly, there was a big blast, with Chibi Chibi as the source. A girl with wings and long hair was floating in the air, speaking to Usagi.

Chibi Chibi: Sailor Moon.
Sailor Moon: Who are you?
Chibi Chibi: I was always looking for someone who would take me in.
Sailor Moon: Who are you?
Chibi Chibi: I'm the glitter that used to be in Galaxia's star, the Light of Hope. Please take me, my glitter of life.
A pink butterfly flew to Sailor Moon, and merged with her star seed.
Sailor Moon: A warm light.
Sailor Moon turned into Princess Serenity. The star seed turned into a sword, similar to the one that Galaxia was holding.
Chibi Chibi: Please take this sword, and defeat Galaxia....