Demetrius in Harpers Dictionary of Classical Antiquities
Of Sunium; a Cynic philosopher, who flourished at Corinth in the first century. During the reign of Caligula he taught philosophy at Rome, where he obtained the highest reputation for wisdom and virtue. He was banished from Rome in the time of Nero for his free censure of public manners. After the death of this emperor he returned to Rome, but the boldness of his language soon offended Vespasian and again subjected him to the punishment of exile. Apollonius, with whom he had formed a friendship, prevailed on Titus to recall him; but under Domitian he withdrew to Puteoli. Seneca, who was acquainted with him, speaks in the highest terms of his masculine eloquence, sound judgment, intrepid fortitude, and inflexible integrity (De Vit. Beat. 25).
Demetrius the Cynic in Wikipedia
Demetrius (Greek: Δημήτριος; 1st century), a Cynic philosopher from Corinth, who lived in Rome during the reigns of Caligula, Nero and Vespasian (37-71 AD).
He was the intimate friend of Seneca, who wrote about him often, and who describes him as the perfect man:
Demetrius, who seems to have been placed by nature in our times that he might prove that we could neither corrupt him nor be corrected by him; a man of consummate wisdom, though he himself disclaimed it, constant to the principles which he professed, of an eloquence worthy to deal with the mightiest subjects, scorning mere prettinesses and verbal niceties, but expressing with infinite spirit, the ideas which inspired it. I doubt not that he was endowed by divine providence with so pure a life and such power of speech in order that our age might neither be without a model nor a reproach.
His contempt for worldly riches is shown by his reply to Caligula who, wishing to corrupt him, offered him two hundred thousand sesterces. Demetrius replied, "If he meant to tempt me, he ought to have tried to do so by offering his entire kingdom."
He was also a friend of Thrasea Paetus and was with him when Thrasea was condemned to death (66 AD). We hear of him again in the reign of Vespasian (c. 70 AD), when, curiously, he defended Publius Egnatius Celer against the charges brought against him by Musonius Rufus. He was exiled from Rome in 71 AD, by Vespasian, along with all other philosophers.
Demetrius is sometimes identified with the Demetrius of Sunium mentioned by Lucian. However, Demetrius was a very common name in the Roman world, and Demetrius of Sunium was probably, (but not certainly), a different, later Cynic.