Theodosius was born a Byzantine subject of Greek origin. He married Agnes, Princess of France, the daughter of Louis VII the sister of Philippe Auguste. Agnes was twice the Empress of the Byzantine Empire, married first to Emperor Alexius II, and then to Emperor Andronicus I.
Theodosius and Agnes were of invaluable assistance to the Crusaders ("Latins") of the Fourth Crusade after they took Constantinople and founded the Latin Empire in 1204. Theodosius was one of the first Greeks to support the Latins and acted as an ambassador to Henry of Flanders when the Greeks of Adrianople and Demotica wished to break their alliance from Tsar Kaloyan of Bulgaria and seek the protection of their cities by the Latins.
Theodosius was Lord of Arpos (which the Latins called Naples of Napoli) and Adrianople.