Underlight was once a fairly normal realm, it's gates were open if not heavily traveled. This was before The City was built, and it had but a few distinguishing features (feel free to add more, but two of these are important to the plot, and the others act as minor and major clues for the quest 'Only the Roses Remember)
Each person in Underlight has an animal, to which they share a special bond. In some ways, the animal is a physical representation of the person's soul. They call them companions, they will recognize the term pet and not immediately correct the usage, but may give the word companion a subtle amount of extra emphasis if the players insist on using pet or some other term.
On naming: If you can think of better names for the background characters, great, but I did use a naming convention. To stress the relationship between a person and their companion, I gave the person a name based on some physical thing, and gave the companion a more abstract name that represented the person's nature (and the companion's.)
The water in Underlight has unusual reflective properties. It reflects light exceptionally well, without causing 'bright spots' where the source is directly reflected. The result is golden lakes, rivers, and seas by day, and silvery ones by night.