Arrian

Greek historian and student of Epictetus who recorded the Discourses and compiled the Enchiridion — and who also wrote the Anabasis of Alexander, the primary source for Alexander the Great's campaigns.

Preserved Epictetus's philosophy for posterity — without Arrian's notes, Epictetus's thought would have been lost entirely, since Epictetus wrote nothing himself — making Arrian the Plato to Epictetus's Socrates.