Justinian I was one of the emperors of the Byzantine Empire. He ruled from 527 to 565 AD. This put him a few decades after the fall of the Western Roman Empire.
For someone who became emperor, Justinian was of very humble background. So much so that very little is known of his youth. What is known is that his parents were Macedonians -- people from an area in the northern part of what is now Greece. Justinian himself was born and raised in what was then called Ilyria. This is roughly the area that is now Croatia.
Ethnically, then, Justinian was Macedonian. Culturally, he was Hellenic, since the Byzantine Empire was by that time very Greek in its culture.