Sei bears the title of "Dragon King of the East", or "Ryū-Ō". He also bears the titles "Prosecutor of the East" and "Young Emissary". He controls the night of Hong Kong and the Special Zone, protecting Red Blood and Black Blood. He's a direct descendant of a chaotic bloodline.
Sei also created the barrier and implemented the restrictions that protects the Special Zone. His eyes are the "switch" for opening and closing the barrier. When he opens his eyes, the barrier opens, and vice-versa. He could turn into an enormous golden dragon made of pure energy.
Before the start of the story, Sei's appearance looked that of a person in his low twenties. Sometime in the past 10 years, he was reborn as a young child. Even though he was reborn into such a "cute" state (as Jiro puts it), Sei is serious and is one of the most powerful vampires in the story, which makes him highly respected. He could easily make a vampire uneasy with his powerful aura. He also has an older sister called the Dark Princess of the North.