Historically, the Holy Roman Emperor is sole Catholic emperor in Medieval Europe. The title is unavailable in the three canonical scenarios of Crusader Kings. However, a kingdom tag for the Holy Roman Empire exists (HREE).
The closest equivalent of the Holy Roman Emperor in Crusader Kings is the Emperor trait. Unlike kings, who were crowned by high-ranking archbishops, emperors were crowned by Popes. To become an emperor, a ruler must either hold five king titles or the king titles of Germany, Italy and Burgundy. He must have at least 10 vassals, must not be modest, and must have considerable piety and prestige. The MTTH for Imperial Coronation is 15 years.