Sei Seragaki is a significant character in the 'Dramatical Murder' series, serving as the twin brother of the protagonist, Aoba Seragaki. Sei was created as a test subject by Toue, enduring harsh experiments due to his unique powers. Despite his frail health, he is gentle, empathetic, and self-sacrificing, often supporting Aoba emotionally and spiritually. Sei's tragic past and deep connection with Aoba make him central to the story’s development and emotional impact.