Who is Roger de Mortimer from The Reckoning and what is their importance?

Roger de Mortimer is a Marcher lord and ally of King Edward. He is responsible for the mutilation of Simon de Montfort's body, and he sends Simon's head to his wife as a keepsake of the battle. Simon's head is put on the gate of Mortimer's Wigmore Castle. Roger de Mortimer causes the first real argument between Llewelyn and Ellen when Llewelyn needs Roger's support against King Edward. Roger's allegiance is for sale to the highest bidder and Llewelyn needs him to win against the Crown.