Who was the Roman emperor defeated by the Muslims in the Battle of Yarmouk, led by Khalid ibn al-Walid?

The Battle of Yarmouk took place in the the year 13 or 15 AH. The commander of the Romans was Theodorus, and the commander of the Muslims was Khalid ibn al-Walid, by order of Abu Bakr al-Siddiq. During the battle, Khalid learned of Abu Bakr's death and Umar's succession, and Umar's appointment of Abu Ubaydah as commander of the army, but he concealed the news. The Muslims were victorious, and Theodorus fled and was later killed. The horses continued to trample the Romans until morning.