Yui is a habitable moon in the Via Sagittaria Galaxy not officially under the control of any nation. It is the headquarters of Dialapse, a large mining company originally owned by the State of Tenshi which has since gained independence. Those living on the moon are mostly nobles in exile from across the Via Sagittaria Galaxy, as its ambiguous membership in any nation allows many to avoid certain punishments or taxes. The planet is claimed by both the State of Tenshi and the United Federation of Star Systems.
The moon is covered in various islands. Every island has a use and was placed mostly intentionally to make the environment appear natural, but also aesthetically pleasing.
Most of the islands have houses or cabins on them for some of the top officials of the Family. Those that have less power live on the central island called "Senta." While living in relatively "cramped" conditions on this large island, they still each have about four square kilometers to themselves. As the population grows, lower-level individuals that are merely employees rather than family members live in apartments on Senta or in orbital infrastructure.
Yui orbits a star which has been noted as very hot and uninhabitable. as such, the different wavelengths given off by that star prevent green plants from thriving. The plants were custom tailored to resemble plants while using purple pigments rather than green ones.
Other than that, the biology is known to be very diverse. They have adapted to live on the low gravity, but since it has't been long, they aren't much different. Flying species have changed the most, generally growing smaller wings and heavier bodies. Insects especially have grown to have much smaller wings to allow for easier burrowing and faster metamorphosis.