Margaret is Robert Haydn's father and the King of the Celestial World Candidate who gives him his power. Margaret 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, it is revealed that his body had been possessed by a Hellion with the intention of taking over Heaven. When he starts to doubt his objectives, he turns into a celestial beast and puts Ueki in his awakening organ (It turns out that his [the hellion] clan had eaten a lot of ancient Heavenly Beasts), and advances Ueki to his 9 and 10 star treasures.