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Even if they saw him slice the head of a demon, demons were able to praise and follow the strong. Fights between clan leaders happened very commonly, so a demon killing another wasn't so much of a shock for them to provoke Erdrich to be instantly made into an enemy. The demons felt confused at first, and the Blue Onis, who were more intelligent than the Imps quickly tried to speak things with him. Despite looking savage and bloodthirsty in the outside, Blue Onis were intelligent and capable of holding conversations with one another to express their opinions… and they wondered why a demon so strong as Erdrich was siding with the humans.
When he heard them calling him a "brother" Erdrich felt completely weirded out and thrown off his own mind. What just happened? They just… really called him a brother? Erdrich couldn't feel any kinship from these monstrous demons at all, and instead felt disgust.
Check "The Protagnist's Sister Is Actually the Strongest"!