Chibi-chibi is the most mysterious, kawaii little girl of the entire series. Her hair is dark pink, and her eyes are deep blue; she's about knee-height to Usagi, and her hair is done in heart-shaped odango. In Act 44 (beginning of Stars arc), she floats quietly down out of the sky into Ichinohashi Park. No one is there to greet her; she climbs up a ramp, and sees Kakyuu lying still on a platform.
The next thing we know, she shows up at the Tsukino household. Usagi's mother opens the door. "Chibiusa-chan!" she cries... then stares down at the little girl. The girl stares back up at her, and gives her the most adorable little smile. "Chibi... chibi?" Tsukino Ikuko says quizzically. And thus, Chibi-chibi is named. (In case you didn't know, "chibi chibi" literally means "small small." Two smalls is tiny.)
For her first couple appearances, she merely repeats what is said to her. However, she comforts Usagi in her own quiet little way, especially as the senshi have their sailor crystals stolen one by one. She also carries a small urn. At the end of Act 45, the Starlight senshi go to the Tsukino household. They demand that Chibi-chibi give them the urn; she shakes her head and cries "No!" over and over again.
But the urn glows, and Princess Kakyuu flows out of the incense burner. She reassures the little girl, and then smiles to the Starlights in welcome. Then Chibi-chibi demonstrates a previously unforeseen power... with the phrase "Chibi-chibi Crystal Power! Make up!" she transforms into a senshi, Sailor Chibi-chibi.
Sailor Chibi-chibi goes with Usagi and the Kinmokusei senshi to Zero Star, where Kakyuu and the Starlights are killed; she supports Usagi through the battles with the evil senshi and the loss of her friends, until Usagi comes up against Galaxia, one-on-one. While taunting Usagi, Galaxia stumbles and falls off the cliff; Usagi grabs her and pulls her back up. When Galaxia asked Usagi why she saved her, Usagi says that she doesn't want to lose any more friends... that there is no one left to fight for. Galaxia says that this must then be the end of war...
Chibi-chibi steps forward and says coldly that the wars and suffering would continue, forever into the future - that the only way to stop them would be for Usagi to destroy the birthplace of the stars altogether. Horrified, Usagi says that if she did, no more stars would be born and that the galaxy would someday die. Chibi-chibi replies that if she didn't, then all the suffering of the future would be on her hands, and that she would come to regret her actions.
Usagi looks at the smaller girl, her eyes warm. "But at the end of war, there is hope and the future," she points out. "Don't give up. Don't abandon hope and the future. Believe. Our star of hope will never disappear. As long as it keeps shining, we'll be all right. We won't be defeated."
Chibi-chibi nods in agreement, hope dawning in her face. Light glows around her, and she becomes a beautiful white-haired woman in a white, ribboned salior fuku. A star glimmers on her forehead, and wings adorn her outfit. She is Sailor Cosmos, the ultimate form of Sailormoon. Tears in her eyes, she leans forward and kisses Usagi softly.
From a future filled with overwhelming, pain-filled wars, she had lost all faith and hope. In that dark time, she remembered the choice she had made in the past - how she had chosen to keep fighting, to keep the galaxy alive. If she hadn't... then the wars wouldn't have happened... so she chose to travel back in time, to make Sailormoon destroy the galaxy.
But Sailormoon was stronger, and made the right choice. And in doing so, she taught her future self the matchless strength that made her the best warrior - the courage to accept everything, and to keep one's hope strong. So Sailor Cosmos headed back to the future, hope once more within her soul.
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