Hadrian is a sixty-year-old Roman Emperor suffering from a dropsical heart. He is a pragmatic ruler with a deep love for Greek art, culture, and philosophy, which earns him the youthful nickname "the Greekling." He favors peace, moderation, and economic stability over military conquest, viewing his role as a public service meant to alleviate suffering. He embraces physical pleasures but values intellectual liberty above all.

Trajan is a celebrated Roman Emperor and military commander who focuses on expanding the empire's borders through conquest in Dacia and the Parthian Empire. He is an active, vigorous leader who values martial prowess. His ambition for continuous warfare places him ideologically at odds with Hadrian, who secretly prefers a strategy of peace and secure borders.

Antinous is a gentle, perceptive adolescent from Bithynia. He possesses boundless youthful energy, great physical beauty, and a deeply superstitious mindset. He travels as Hadrian's companion and lover, showing absolute devotion to the emperor and bringing about a period Hadrian considers a golden age.

Plotina is the Empress of Rome and the wife of Trajan. She is a chaste, soft-spoken, and calm woman who adheres to Epicurean philosophy. She acts as a wise counselor and a trusted political ally to Hadrian, offering him a rare platonic friendship with a woman whom he deeply respects.

Arrian is a military officer, political operator, and philosopher from Nicomedia. Following Stoic principles, he lives a life of moral discipline without self-righteousness. Hadrian admires his sense of moderation and treats him as a close peer and equal within the administration.

Servianus is an older Roman politician and Hadrian's brother-in-law. He is a grasping, self-important man who frequently opposes Hadrian's rise to power. His resentment and ambition make him a persistent political antagonist within Hadrian's own family circle.

Mark is a seventeen-year-old youth and the adopted grandson of Hadrian. He is the intended recipient of Hadrian's long, reflective memoir. He displays a sober, intelligent nature that blends sweetness and severity, marking him as a promising future leader in the eyes of the aging emperor.

Sabina is Hadrian's wife and the Empress of Rome. Their marriage is a cold, bitter arrangement, and they largely live apart. She represents a strict, unloved version of matronly virtue and often surrounds herself with her own literary and social circles.

Attianus is a powerful political figure who serves as Hadrian's mentor and former guardian. He is fiercely protective of Hadrian's position and is willing to employ violent tactics to secure the emperor's reign, operating outside Hadrian's preference for justice and peace.

Lucius is a young Roman noble who catches Hadrian's attention. He is brilliant and physically beautiful, but also frivolous and prone to accruing debt. He represents the kind of youthful appeal to which Hadrian is naturally drawn.

Antoninus is a mature, honest, and dignified Roman senator. He is an upstanding administrator whom Hadrian trusts to continue his policies and preserve the stability of the empire for the next generation.

Phlegon is Hadrian's secretary. He assists with the emperor's daily correspondence and writes historical accounts of the empire, serving as a reliable and literate administrative aide.

Chabrias is a tutor employed to educate Antinous during the emperor's travels. He becomes deeply devoted to both the young man and Hadrian, remaining a loyal member of their inner circle.

Hermogenes is Hadrian's personal physician. He diagnoses the emperor's dropsical heart and attempts to provide care, though his treatments are ultimately limited by the reality of human mortality.

Euphorion is an enslaved servant who has attended Hadrian's bath for twenty years. Despite their daily physical proximity, his inner life and personal thoughts remain a complete mystery to the emperor.

Marullinus is Hadrian's grandfather, a man of senatorial rank who served under Titus. He is a hard man interested in astrology who predicts early on that Hadrian is born to rule.

Simon Bar-Kochba is a Jewish man who proclaims himself the Messiah and leads a revolt against Hadrian's forces. He actively opposes the Roman attempt to assimilate Judea, drawing widespread local support.

Akiba is a delegate and leader within the Jewish community who firmly rejects Hadrian's efforts to assimilate them into Roman culture, representing a religious resistance to imperial expansion.

Apollodorus is the skilled architect tasked with designing the Pantheon. He frequently clashes with Hadrian over aesthetic and structural choices, showing a willingness to argue with the emperor.

Euphrates is a Stoic philosopher who approaches Hadrian to ask for permission to die by suicide, presenting a sharp contrast to Hadrian's own clinging to worldly possibilities and experiences.

Osroës is the Emperor of the Parthian Empire. He stands as a major foreign adversary to Rome, using rebellion and strategic attacks to resist Trajan's campaigns of conquest before eventually negotiating with Hadrian.