Yuki Onna (雪 女) is an ice planet, and the fourth planet in the Byakko System. Yuki Onna is mostly uninhabited, apart from several research stations located at the poles of the planet. The planet is notable due to its many undiscovered Ancient Alien Tombs located under the planets surface. Yuki Onna is one of the most distant planets from the centre of known space. Yuki Onna gains its namesake from the Japanese folklore creature, the Snow Woman, due to the planet's continuous snow and ice cover.

Before humans settled the region of space, the planet, like almost every other in known space, was inhabited by the ancient race of aliens, known only as 'those who came before.' Many speculate the planet to have been a burial world for the civilisation's greatest, due to its many underground tombs. The amount of tombs, has made it a hotbed for pirates, and tomb raiders.

In 2899 the Crew of the Ryujin discovered the first segment of the map, unknown to them at the time, in one of the planet's many derelict underground tombs. Taking the segment meant that the tomb's booby-traps were set off, however the crew escaped unscathed. They then left the planet to take the unknown artefact to Kazuo Tsutomo, and elderly alien artefact specialist, who informed the crew that it was part of a greater treasure map with pieces scattered across space.