Constantine the Great
Constantine the first was Roman emperor between 306 and 337 CE. He effectively ended the system of four emperors known as the Tetrarchy, defeated all pretenders and established an undisputed sole rule over the empire. While respecting the traditional pagan rituals, he was the first Roman emperor to openly support Christians and was even baptised shortly before his death.
Among the most defining moments of his life, one could count:
-defeating his rival Maxentius in the Battle of the Milvian Bridge (carrying Christian symbols)
-defeating Licinius, his co-ruler and gaining unopposed authority over the whole empire
-founding Constantinople, the "New Rome" and establishing his court there
-organizing the Council of Nicea to resolve theological differences of the Christians (mainly to deal with Arianism)
-creating a new gold coin known as "solidus" and thus improving the economic situation of the empire.