Epictetus (50-135 AD approx.) was a Roman philosopher of Greek-Phrygian origin. He is often considered the central teacher of Stoicism in Rome, succeeding Seneca and preceding Emperor Marcus Aurelius in the big three philosophical figures of the Roman Empire.
Far from the stereotype of the ancient Greek thinkers being slovenly and well off, Epictetus was born into slavery and passed most of his life in it, only managing to become a philosopher because urban slaves were generally allowed to have their own lives and jobs. Originally hailing from Hierapolis (currently Pamukkale, Turkey), he belonged to Tiberius Claudius Epaphroditus, a secretary of Emperor Nero under whose permission he started studying Stoic philosophy as a child. Their mutual connection to Nero means there is a non-zero chance Epictetus crossed paths with Seneca, but he learned mainly under Musonius Rufus, another celebrated Roman teacher of virtue, until being freed at eighteen years old and allowed to open his own school.
He later moved his business to Nicopolis, Greece when Emperor Domitian expelled all the philosophers from Rome for being unsubmissive (and executed Epaphroditus for assisting Nero's suicide), although they were rehabilitated by the next emperors, especially the Hispanic duo of Trajan and Hadrian, who loved Greek philosophy and cultivated connections with Greece. Hadrian in particular became friends with Epictetus, ironic as it was for an infamously loony emperor (some believe he was actually seeking out Epictetus as we would seek a therapist), and a later work registering a supposed long talk between the two became very popular the next centuries. Epictetus died at around eighty-five, leaving an adopted son behind and passing the torch to his major apprentice Flavius Arrian, of whom Marcus Aurelius' Stoic mentor Junius Rusticus was either an apprentice or a classmate.
Epictetus is easily the most hardcore and vibrant Stoic of the Roman era. He was apparently an incredible speaker, who could entrance people with a few minutes, but he didn't only teach philosophy — he lived, breathed it, since the first moment he knew it. He cultivated an iron-hard sobriety for all of his life and never gave a crap about being slave or free, to the point he never bothered to change his slave name despite "epictetus" not being actually a person's name but the Greek word for "acquired". For Epictetus, freedom is within us, not outside, and cannot be taken from us if our minds are strong and virtuous. He had also a lame leg, presumably from birth (a Christian legend claims that Epaphroditus was a jerkass and crippled him as an adult For the Evulz, although this sounds more like an embellishment), but always made a point about how little a wretched body could hinder a mind in tune with reason.
He left no writings, but Arrian compiled a list of short teachings of his, the Enchiridion or Handbook, and another list of polemics, the aptly named Discourses, which nowadays are often published together. Epictetus' philosophy as contained in them is a standard if zealous variation of Stoicism, emphasizing the premises that it is our mind which disturb us, not the things outside, and that we should desire things as they happen, not as we want them to be. Also, although he shared the usual Panthetistic outlook of the Stoic universe, he was interestingly severe about the importance of traditional worship, society and traditions, to the point he could be considered a Roman conservative and a sort of pagan crusader. This makes him the Arch-Enemy of Epicureanism, whose tenets he considered terrible, counterproductive and hypocritical despite the presence of high similarities in other aspects.
This harsh, ascetic outlook might not make Epictetus the most relatable Stoic, as he sometimes reads like a quite impatient mentor and makes it sound really easy to have a monk-like discipline like his, but there is still something really admirable and vital on him, which has helped him survive the centuries and remain relevant today. Psychologists Aaron T. Beck and Albert Ellis, founders of the now-ubiquitous cognitive behavioral therapy (CBT), cited Epictetus' writings as a vital inspiration for them.
