His rank is that of a Cardinal, and he craves for the position of Pope, which he obtains at the end of World War III, though of a weakened Roman Catholic Church.

He later succeeds Matthai Reese as the Pope of the Roman Catholic Church, though has already become a much nobler person after the War.

Unlike his predecessor Matthai Reese Pietro seems to be more for the power of the pope, and doesn't seem to be very caring of other people. While he is similar to Nikolai Tolstoy in that perspective but, unlike him he neither worked with Fiamma or cooperated with him in any way though that did not stop him from taking advantage of the situation.

(Source: Toaru Majutsu no Index Wikia)