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The Sin Sack is a faceless male mob character originated from Touhou fandom. They wear a sack over their head marked with the character 罪 (Tsumi, or "Sin"). As a result, they are referred to as 罪袋/Tsumi-fukuro, which literally means "sin bag" or "sin sack".
They are frequently depicted as perverted followers of various female characters. In recent times, they are most commonly seen on various NSFW MMD videos (particularly those from the Iwara.tv website), where they are used as a generic male mob characters.