who was the first holy roman emperor?

Question

and why was charlemagne significant in the holy roman empire?

Answer 1

Charlemagne, King of the Carolingians, claimed to be the sucessor of the Western Roman Emperors, but the Holy Roman Empire did not come into existence until long after his death, at the coronation of Otto I (962).

Answer 2

you answered the first part of your question the answer to the second part is that he recreated the roman empire. not as it was at it's greatest but he carved out quite an empire.