The Sin Sack is a faceless male mob character originated from Touhou fandom. They wears 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 a perverted followers of various female characters. In recent times, they are most commonly seen on various NSFW MMD videos (particularily those from the Iwara.tv website), where they are used as a generic male mob characters.
