As far as I understand it, Petrus Romanus will be the Pontiff who works hand in hand with the Great Monarch to set things straight. IMO, he probably reigns the entire time the GM reigns, and is the last Pope. Then, Anti-Christ, who may very well wound the GM mortally in the great battle at Megiddo (where, btw, Josiah was wounded mortally, going to the Mount of Olives to die). That is my take, anyway.