Robert Haydn is a frightening person with an equally frightening power. He can boost his own natural sacred treasures by modifying them to a perfect ideal like absolute accuracy or ideal protection. The fault in this is that for every time he creates an ideal, he loses a year of his life, but once he creates this ideal he can use it any time he wants to. His abilities can't be used on living things, however, but his changes change as his ideals change, such as turning his Kurogane rapid-fire and/or unblockable. His level 2 power allows him to change gravity, and he can also make soap bubbles that can reverse gravity too.
That is also why the reason he forms Robert's Ten. They act as his brawn, doing the fighting for him in order to save those precious years. He is a fairly nasty character, not caring who he harms and how badly they are harmed. Like Ueki, he is a Heavenly Being with more power than the gift given by his King of the Celestial World Candidate.
But unlike Ueki, however, Robert has been training in his power for quite a long time. Robert has an unfortunate past in which he was treated as a monster by the people of his town. After being betrayed by his friends, he starts to hate humanity and destroys the entire village where he lived (1/3 in the manga) and gets eaten by Anon.
At the end of the series, he becomes a nice person like his childhood self, having been touched by Ueki and his friends