This moon was meant to be a test to see whether ocean life from Europa and Earth could live together and create a balance. After much trial and error a balance was established.
Surprisingly the Kya and dolphin population demonstrated an unusual amount of teamwork when hunting prey. Dolphins became like horses to the Kya, which prompted the Europan Kya to ask for dolphins as well. The amount of mutual cooperation shocked many scientists and humanity as a whole.
When humans built floating cities, they tried their best to fit into the new ecology and eventually succeeded in achieving a mutual understanding with the Kya and the dophins.
The human population of Callisto is 340 million, while the Kya population is just over 100 million and the dolphin population is 97 million.
Most humans live on the surface in floating cities and islands but some merefolk from Ganymede moved and built small towns underneath the surface of the moon.
Like most oceanic world, its climate is very warm and predictable as there are no land masses to block air currents.