Sailor Cosmos is, in the manga, the ultimate form that Usagi takes in the future. She is by far the prettiest form she takes. I love her heart shaped odangos. The fuku is beautiful too, with its winged medallions and the long cape. But I'm totally mad about the shoes. They are gorgeous.
In the anime she does not exist. She is often confused with Sailor Galaxia, the Legendary senshi.

Sailor Galaxia, before Chaos possed completely her body released her starseed, the very essence of souls. After a fight against Galaxia, every senshi, except the Starlights and Moon were killed. In order to help Eternal Sailor Moon defeat Chaos, Sailor Chibi Chibi who was Galaxia's starseed, transformed into a heart-shaped-odango shadow: the anime Cosmos-like figure.

• Story cleared up
• Lost senshi