Are there any myths about him? I can't find much about him besides the fact that he became a demonic god at some point.
Tengri, the chief deity of the Turkic pantheon, created Er Kishi to serve as his companion:
In Turkic mythology, Tengri is a pure, white goose that flies constantly over an endless expanse of water, which represents time. Beneath this water, Ak Ana ("White Mother") calls out to him saying "Create". To overcome his loneliness, Tengri creates Er Kishi, who is not as pure or as white as Tengri and together they set up the world. Er Kishi becomes a demonic character and strives to mislead people and draw them into its darkness. Tengri assumes the name Tengri Ülgen and withdraws into Heaven from which he tries to provide people with guidance through sacred animals that he sends among them. The Ak Tengris occupy the fifth level of Heaven. Shaman priests who want to reach Tengri Ülgen never get further than this level, where they convey their wishes to the divine guides. Returns to earth or to the human level take place in a goose-shaped vessel.
Wikipedia contributors. (2020, December 14). Tengri. In Wikipedia, The Free Encyclopedia. Retrieved 13:46, January 7, 2021, from https://en.wikipedia.org/w/index.php?title=Tengri&oldid=994196124
There's a poetic adaptation of the Turkish Creation Story by Eugene Doty you may find interesting.