Koshey has magic as his disposal. In some tales, he can cast a sleep spell that can be broken by playing enchanted gusli. He may ride a three - or seven-legged horse, may have tusks or fangs, and may possess a variety of magical objects like cloaks and rings that a hero is sent to obtain. He may have other magical powers as well. In one tale, he has eyelids so heavy that he requires servants to lift them. In others, he has shape-shifting abilities, but only with regard to birds (birds seem to be able to traverse from underground/other realms to the earthly realms). In a way, he represents mortality.
It is believed that, like Baba Yaga is the maternal figure to heroes and heroines, Koshey is the father figure. He represents the barrier or test that a couple must pass before they are betrothed and wed. He may initially be the benevolent, but later, the malevolent father to the bride. Koshey and Baba Yaga frequently do not appear in fairy tales together; however, when they do, they can be lovers, a married couple, or siblings. Either way, their relationship is shrouded in mystery!
Koshey Bessmertny, Wikipedia
Vladimir Propp's folktale morphology
Baba Yaga: The Ambiguous Mother and Witch of the Russian Folktale, Andreas Johns
Russian Fairy Tales, Aleksandr Afanasyev
Alkota thanks very much Ms. Olesya Salnikova Gilmore, the Russian folklore analyst and fairytale researcher for preparing this information.