Sailor Cosmos was a pivotal character in the Stars arc of the Sailor Moon manga. She was revealed to be the true form of Chibi Chibi, whose origins differed significantly in the anime. After Sailor Galaxia's demise, Chibi Chibi disclosed to Sailor Moon that she was actually Sailor Cosmos. The future, as described to the Sailor Quartet, was bleak—Sailor Chaos waged a devastating war, annihilating everything and everyone Sailor Cosmos knew. Overwhelmed with fear and despair, Sailor Cosmos fled to the past, intending to destroy Chaos before it could achieve its formidable Senshi form.
To defeat Chaos, Sailor Cosmos believed she needed to destroy the Galaxy Cauldron. As Chibi Chibi, she attempted to guide her past self, Eternal Sailor Moon, towards this goal. Despite Eternal Sailor Moon's determination to save the Cauldron and choose a different path, her courage inspired hope in Sailor Cosmos. It was stated that the Eternal Sailor Moon who saved the galaxy was indeed the true form of Sailor Cosmos, and only when she embodied the same strength and courage would she truly become Sailor Cosmos.
After sending the Sailor Quartet to the future in Act 52, Sailor Cosmos' fate was left unspecified. She expressed her intention to return to her own future, inspired by Sailor Moon's strength, and resume her fight to reclaim her lost mantle due to her cowardice.
The nature of Sailor Cosmos remained ambiguous. Her appearances in Acts 51 and 52 raised more questions than answers. Sailor Ceres mentioned that she was the ultimate future form of Sailor Moon, but whether Sailor Cosmos was the future self of the current Sailor Moon, a reincarnation, a descendant, or an alternate future version was unclear. Naoko Takeuchi suggested that Sailor Cosmos' true identity was meant to be ambiguous, even to herself.
In the final act of the manga, Sailor Cosmos reveals herself from a future ravaged by Chaos. Despondent after eons of battle, she travels back as the infant ChibiChibi to encourage Eternal Sailor Moon to defeat Chaos. Initially, she wanted Sailor Moon to destroy the Galaxy Cauldron to ensure Chaos' destruction, but Sailor Moon protested, understanding that destroying the Cauldron would prevent new stars from being born. Instead, Sailor Moon chose to sacrifice herself to the Cauldron, a decision Sailor Cosmos eventually realized was correct. Reminded of the strength and courage she needed, Sailor Cosmos returned to the future with renewed hope.
The manga implies that Sailor Cosmos is the ultimate form of Sailor Moon, confirmed in an interview with Naoko Takeuchi. Various theories exist within the fandom regarding what this means. Sailor Cosmos never appeared in the anime series, though the silhouette of ChibiChibi's adult form, the "Light of Hope," closely resembled her. Naoko Takeuchi expressed her wish that Sailor Cosmos had been included in the anime.
