Cosmog resembles a floating cloud colored in dark blue and purple. Two light-blue cloud-like formations stem upward from Cosmog's body and seem to be similar to a pair of arms. It has a gold crescent-shape around its body and its face is black with blue cheeks.
As seen in Lillie's Cosmog, Nebby, it appears to be a curious and playful Pokemon. It also is overly trusting and naive, carelessly approaching even people who may not have the best intentions.
In the anime, Nebby is shown to have traits similar to newborns as it tends to cry and sleep a lot. Nebby is also shown to be a picky eater and only seems to eat konipeto.
Under stress and pain, Cosmog can create Ultra Wormholes in an attempt to escape its tormentor(s).
Cosmog evolves into Cosmoem, which evolves into a different Pokémon depending on the version of the game. If it evolves in Pokémon Sun or Ultra Sun, it will become Solgaleo; whereas if it evolves in Pokémon Moon or Ultra Moon, it will become Lunala.
A Cosmog named Nebby features prominently in the games' main plot, under the ownership of Lillie. A running gag in the story is Nebby frequently escaping from Lillie's handbag and getting itself into trouble.