Who is Guy de Montfort from The Reckoning and what is their importance?

Guy de Montfort is Simon de Montfort's son. and Vicar of Florence. He fights with his father in the battle of Evesham and blames his Bran for the battle's failure. Like his brother, he is haunted by images and ghosts from that battle, and when Hal appears in Italy, Guy is overcome by the desire to kill him the man who betrayed his father in the midst of battle.