Frederick II (1194-1250) was the son of the Emperor Heny VI and the Norman queen of sicily Constance. So he was at the same time King of Sicily and Emperor of Germany. Born in central Italy, he had German, Sicilian, Viking and French blood.
He considered himself the successor of Roman Emperors of antiquity, but in reality acted as a proto-modern monarch establishing the first state university and abolishing the trial by ordeal. Deeply catholic, he was nonetheless influenced by the Muslims of sicily and entered in philosophical debates with the Islamic scholars of the Mediterranean. Fibonacci, the mathematician that brought Arab numerals to Europe, was at his court.
Since the HRE included northern italy and Sicily controlled southern Italy, the Popes reluctanctly agreed to have the Papal states encircled by his dominions. In exchange, they obtained the promise by Frederick to organize a crusade to recapture the Holy Land. And yet Frederick was hostile to enter a religious war with the muslims, so he obtained the crowns of Jerusalem, Nazareth and Betlehem, via marriage and diplomacy rather than conflict.
At the peak of his power he controlled much of central Europe and almost all of Italy. Subutai and the Khan asked him in an embassy to sumbit all of his territories to the Mongols and he refused.
Called the "Anti-christ" by his enemies and the "Wonder of the World" by his supporters, he was excommunicated three times and de-excommunicated twice.
Frederick was viewed as an Italian King by the German princes and as German Emperor by Italian republicans. He lost his power in the context of civil wars and rebellions with the cities of the Empire. His territories were divided by his heirs and enemies.
Source: reddit post