The first time Chibichibi transforms into a senshi is when Aluminum Siren tries to steal Eternal Sailor Moon's brooch. When she touches the tier on the ground, a burst of pink light washes over where the fight is taking place. Eternal Sailor Moon is then released from Aluminum Siren's hold and finds herself facing a fuku-clad toddler whom she begins to call Sailor Chibichibimoon.

But that isn't really Chibichibi's true identity. Her identity isn't revealed until the very end... when Eternal Sailor Moon and the Starlights are now fighting Galaxia.

On the very last episode of the series, Sailor Chibichibimoon restores Usagi's crushed starseed back into her body. That's when Galaxia questions herself whether the power emanating from the kid is the only one who can actually defeat her.

When Usagi regains consciousness, she sees a silhouette of a winged young woman who sports a similar hairstyle. The woman introduces herself as the starseed, the Light of Hope, that was once within Galaxia. By turning into her original form, she offers her 'shining life' to Usagi who then transforms into Princess Serenity. She pleads for Serenity to finally defeat Galaxia by using the Sword of Sealing.

Of course, Galaxia tries to inflict damage on Serenity. But since the latter is reluctant to fight, the Light of Hope tells her the truth about the legendary senshi. That Galaxia, upon sealing Chaos within her body, began to turn evil herself. Before her condition got worse, however, she sent her pure starseed into the far reaches of the galaxy where it eventually reached Usagi.

Yeppers, you got it right: Galaxia's starseed is actually Chibichibi. But that's not all. There's more!

Galaxia gets the upper hand when she destroys the Sword of Healing. The pieces of the broken sword revert into Chibichibi who then shimmers out of existence. She doesn't come back until Serenity heals Galaxia and banishes Chaos back to everyone's minds.

When she comes back, she is being held by Prince Endymion. Actually he says that Chibichibi is the one who guided him back to Usagi. A few moments later, Chibichibi thanks her and she disappears in a flurry of light (to follow Galaxia and the other starseeeds, of course).