Once you beat a demon you'll be able to form a pact with him which allows you to play in his human form. There is a catch of course: it costs $2.99 of real money to form a pact and unlock the character.
The sequel to the award winning Hellblade: Senua’s Sacrifice, Senua returns in a brutal journey of survival through the myth and torment of Viking Iceland. Intent on saving those who have fallen victim to the horrors of tyranny, Senua faces a battle of overcoming the darkness within and without.