Rusky Lejon is originally a character from The Brothers Lionhart by Astrid Lindgren. 

In the adaptation for the Masculine War, Rusky is a gunsman from Howling Voice Guild just like his adoptive brother, Jonatan Lejon. He sees his brother being sent to Higheast and being branded by heretic after. He accepts the job to hunt Jonatan to learn the reason behind his brother's rebel. He heads south for Higheast.