Margarette is Robert Haydn's father and the King of the Celestial World Candidate who gives him his power. Margarette sends his own son to Earth as a child so that he could choose him for the tournament. He is not above sneaky tactics to ensure his son's victory.
Later, his body is possessed by a Hellion with the intention of taking over Heaven. In this new form, he puts Ueki in his awakening organ (It turns out that his clan had eaten a lot of ancient Heavenly Beasts), and advances Ueki to his 9 and 10 star treasures