Short answers, pulled from the story.
Mardonius was a Goth-born Roman rhetorician, philosopher, and educator who served as the childhood tutor of the 4th century Roman emperor Julian. He grew up as a slave in the household of Julius Julianus, the praetorian prefect of Licinius, and was trained there as a scholar.
Mardonius first served as tutor to Basilina, the daughter of his master Julius Julianus. Basilina married Julius Constantius in 330 and had one son, the future emperor Julian. After both Basilina and Julius Constantius died, Julian lived with his grandfather from 338, with Mardonius as his tutor.
Mardonius introduced Julian to Socrates, Plato, Aristotle, and the Homeric epics, and defied an imperial decree from Constantius II that prohibited Julian from studying ancient Greek philosophy. Julian later credited Mardonius as "the most responsible of all for my way of life." Historians believe Mardonius's influence may have contributed to Julian's later decision to abandon Christianity.
In the Misopogon, a satire Julian wrote about himself, Julian called Mardonius "this old man" and described how the great philosophers had first convinced Mardonius, who then passed that conviction to Julian. Julian wrote that striving to imitate those philosophers would make him "better than my former self."
Mardonius centered his teaching on Homer and the Homeric virtue of simplicity, which he set against the decadence of contemporary Roman society. Because of his contempt for entertainment, Julian did not attend theaters or horse races until adulthood. The scholar Polymnia Athanassiadi described Mardonius's ambition as making "the young prince able to appreciate the grandeur of simplicity."
After Julian came to power, Mardonius became one of his advisers and frequently visited him for dinner in Constantinople. Julian had called the separation from Mardonius at age 11 one of the most painful events of his life. The historian Giuseppe Ricciotti wrote that among all Julian's memories of violence and outrage, only that of Mardonius came to the emperor's mind "like a beam of light shining in darkness."