All three characters were introduced in the 5th and final story arc of the manga, Sailor Stars. Chibi-Chibi and Sailor Galaxia are in the Sailor Stars TV series as well, but Sailor Cosmos did not appear in it. (At the most, she was reduced to a 30-second scene where she was in shadows and never named) There is a LOT of confusion about Chibi-Chibi’s origins, because the manga and the anime gave her two very different backstories. And there’s even further confusion about Sailor Cosmos herself, because Naoko Takeuchi intentionally made her character ambiguous. Keep in mind that although the anime and manga have the same characters, they are two separate universes, and should not be combined. So, let’s try to clear a few things up.
WHO IS CHIBI-CHIBI?
Chibi-Chibi (or ChibiChibi/Chibi Chibi if you prefer, doesn’t matter) is a tiny girl physically around the age of 2 who floats down from the sky to 20th Century Tokyo, while using an umbrella. She cannot speak much; she usually repeats what other people say, and communicates through body language and physical gestures. Like Chibiusa, she has a mysterious power that hypnotizes Ikuko Tsukino into thinking she’s her daughter, so Usagi must watch over her as an older sister. Usagi is uneasy at first, but eventually doesn’t mind too much because Chibi-Chibi is much more sweet-natured than Chibiusa was.
It is eventually revealed that Chibi-Chibi is not all that she appears to be (more on that later) as she possesses the ability to transform into a Sailor Senshi. In the manga, she’s “Sailor Chibi Chibi,” in the anime, Eternal Sailor Moon calls her “Sailor Chibi Chibi Moon.” (Which is technically incorrect since Galaxia is not related to the Moon, but Sailor Moon didn’t know that yet) In the anime, Chibi-Chibi lends some of her power to Eternal Sailor Moon, who upgrades her “Starlight Honeymoon Therapy Kiss” attack to “Silver Moon Crystal Power Kiss.”
What is important to know about Chibi-Chibi is that she is NOT Usagi’s daughter, Chibiusa’s daughter, nor Princess Kakyuu’s daughter. Sailor Pluto establishes that Chibiusa is Usagi’s only daughter, and that the Silver Crystal is passed down through a single (so far) unbroken line of women. In the manga when Chibiusa meets Chibi-Chibi, she explains that she feels no connection to her. And as for Princess Kakyuu, she has an entirely different story that is unrelated to Chibi-Chibi’s.
WHO IS SAILOR GALAXIA?
Sailor Galaxia is one of the most powerful Sailor Senshi, and the primary villain for the Stars story arc. In the manga, she explains that she was born on a planet she described as “trash,” discovered that she had the power of a Sailor Senshi, and decided that she had to become the most powerful Senshi in the galaxy. In the anime, she was already the most powerful Senshi in the galaxy, but it wasn’t until Chaos (the origin of evil in the universe) corrupted her that she turned evil. Galaxia began traveling from planet to planet and destroying them to gain access to Pure Star Seeds (or Sailor Crystals in the manga). Those who possess Pure Star Seeds are generally Princes, Princesses and Sailor Senshi. It is an understatement that she has a lot of blood on her hands.
She came to Earth not just to take the Star Seeds/Sailor Crystals of the Senshi (and Prince) who lived there, but also to use Eternal Sailor Moon to defeat Chaos for her. She hoped that by breaking Sailor Moon’s spirit, Chaos would try to take her over, and their opposing powers would cancel each other out and destroy both beings. Instead, while Chaos was empowered by Sailor Moon’s grief, it used this power not to attack Moon, but Galaxia, by trying to throw her into the Galaxy Cauldron (the birthplace of the stars). Sailor Moon tried to save her, but Galaxia was overwhelmed by Sailor Moon’s healing powers and her love, and she lost her will to fight. Her bracelets shattered, and she disappeared.
Now, how does she relate to Chibi-Chibi? Well, their origin story in the anime is one and the same. In the anime, Sailor Galaxia was originally a good Senshi, albeit a proud and extremely powerful one. Unable to defeat Chaos herself, she sealed it within her. She knew that Chaos was going to take over her soul, so she sent away her own Star Seed, the Light of Hope, to where it could be found by someone who would embrace it warmly. That Star Seed was Chibi-Chibi. She awakened as The Light of Hope when it seemed that Galaxia was going to defeat Sailor Moon. Chibi-Chibi became a shadowed woman strongly resembling Sailor Cosmos, who urged Sailor Moon to fight back against Galaxia. She turned into a sword intended for doing battle with Galaxia. Sailor Moon was unwilling to fight, and Galaxia shattered the sword, making Chibi-Chibi disappear as if dead. Instead, Sailor Moon used her healing power to break through Chaos’ influence to reach the good part of Galaxia that was still inside her. Galaxia, now healed, promised to repair the damage she made and return all the Star Seeds that she stole. Just before departing, Chibi-Chibi led Prince Endymion’s spirit to Usagi. She thanked Usagi, then went back to join Sailor Galaxia in her mission of reparation.
WHO IS SAILOR COSMOS?
Before I start, please understand that you are not supposed to completely know who this character is, as she is intentionally ambiguous (even Takeuchi-sensei doesn’t really know). In the manga, Sailor Cosmos is Chibi-Chibi’s true form. She comes from a dark future where Sailor Chaos has completely destroyed the universe beyond repair. Terrified and hopeless, she ran away from her future to join Eternal Sailor Moon in her present. Cosmos believed that the Galaxy Cauldron needed to be destroyed to defeat Chaos. Eternal Sailor Moon refused to sacrifice the Cauldron, and resolved to find another way to defeat Chaos. Cosmos is inspired by Eternal Sailor Moon, and returns to the future to continue battling, with the implication that Eternal Sailor Moon’s choice not to destroy the Cauldron means that Chaos may rise again.
As for who Sailor Cosmos’ true identity is, a clue is given when Sailor Ceres asks her if she is the “ultimate” future form of Sailor Moon. This could mean many things, and none of them are answered for sure.
A few possibilities:
#1, She is Usagi, and an incarnation of Sailor Moon that she achieves *after* becoming Neo-Queen Serenity (which contradicts the fact that Neo-Queen Serenity lost her Senshi powers after ascending to the throne)
#2, She is Usagi, and an incarnation of Sailor Moon that she achieves *instead* of becoming Neo-Queen Serenity (so Cosmos comes from an alternate dark future)
#3, She is Usagi, but a reincarnation of her rather than the future form of the Usagi we know.
#4, She is a descendant of Usagi who now bears the Sailor Moon title – not Chibiusa though, because as mentioned before, she feels no connection to her. Some evidence for this is that the Silver Crystal extends the current owner’s life a maximum of 1000 years, and Sailor Cosmos clearly comes from an era long after that.
You are intended to draw your own conclusions and theories about Sailor Cosmos. (I go for theory #2 myself) As for the anime, she is not a character in it, and her only appearance is an ambiguous one after Chibi-Chibi becomes the Light of Hope. An animation cel of the moment reveals that The Light of Hope’s design is definitely based on Sailor Cosmos’: http://silvermoon424.tumblr.com/post/54723569958/an-animation-cel-of-the-light-of-hope-as-you-can