Empress Irene of the Byzantine Empire is the one in the corner, who claimed to be Emperor of the Roman Empire.
Emperor Charlemagne of the Holy Roman Empire is the one being crowned, and claimed to be the Emperor of the Roman Empire. The Pope is crowning him, because the Pope claimed to have the authority to appoint the Emperor of the Roman Empire.
The Roman Empire itself is not pictured, because it fucking fell almost 300 years before any of this, despite the claims of everyone who wanted a taste of Imperial legitimacy.
The two Abbasid guys are just chillin'.