As the emperor whose reign precedes his distant kinsman Hadrian’s, Trajan serves as a foil to Hadrian. Trajan is a man of action who values vigor and conquest more than peace or justice, and most of his life is focused on expanding the Roman Empire. Hadrian disagrees with Trajan’s ambitions, but makes the strategic choice to keep his opinions to himself; he publicly supports Trajan, fulfilling his civil duties as assigned and taking part in military action in Dacia at Trajan’s request.

Trajan is an example of a warrior king, as opposed to Hadrian’s effort to be a Platonic philosopher king. Trajan also plays an antagonistic role: He shows occasional dislike for Hadrian and, at one point, they compete for the same love interest. Trajan shows a desire to hold onto his power by refusing to officially adopt Hadrian, although he gives the younger man a ring that the Emperor Nerva gave to Trajan. Hadrian sees it as his duty to give Trajan full imperial honors at his death, recognizing his legacy, even as he decides to set a much different stamp on his own reign.

Antinous has an important influence on Hadrian as first a love interest and then a monument to which Hadrian does ongoing homage. Antinous is based on the historical young man who was known to be Hadrian’s companion and lover, but Yourcenar adds her own interpretation in making Antinous somewhat dreamy, both energetic and detached, and inflexible in his beliefs, loves, and requirements.

Antinous’s father held a post in civil administration in Bithynia, leaving the boy in the care of a grandfather, who sent him to study in Nicomedia, where he is living with a friend of the family, a shipbuilder, when Hadrian meets him. Hadrian calls him “childishly trusting” but “also disposed to reflection” (154). Hadrian sees in Antinous a mixture of qualities he characterizes as Greek and Asian, with “mystic superstitions like those of a disciple of Apollonius, and the religious adoration, as well, of an Oriental subject for his monarch” (155). He notes the young teen’s silent manner and the way he follows like a “familiar spirit” or a “graceful hound, [who…] took his post at my feet” (155). The narrator describes the young man as having “the infinite capacity of a young dog for play and swift repose, and the same fierceness and trust” (155).

Antinous’s devotion gratifies Hadrian, but it also proves irritating. Antinous wishes to be monogamous while Hadrian doesn’t. For example, Hadrian suspects that Antinous is jealous of Lucius, to whom Hadrian is also attached. Antinous is portrayed as credulous, swayed by omens and superstitions; Hadrian supposes the boy dies by suicide in an effort to magically add the years of his unlived life to Hadrian’s.

After Antinous’s death, Hadrian’s effort to build a cult of remembrance allows different communities across the empire to reinterpret Antinous, sometimes as a hero, sometimes as a young god, and sometimes as a protective spirit. His death leaves an indelible imprint of grief on the narrator.

Plotina is a minor supporting character and one of the few women characters in the novel. As the wife of Trajan and eventual empress, she serves as a mentor and ally for Hadrian; she is one of the few women he admires and one of the few friends with whom he can speak freely and be understood.

Plotina serves as a model for the virtuous Roman matron: chaste, calm, docile, soft-spoken, and self-possessed. Hadrian notes her restraint of speech, observing that her words were “so measured as to be never more than replies, and these as succinct as possible” (81). She dresses simply and projects a demeanor that is calm and soothing. Plotina favors Epicurean philosophy, which Hadrian admires as “that narrow but clean bed where I have sometimes rested my thoughts” (81). Hadrian suspects that Plotina plays a role in producing the official documentation of Trajan’s adoption of Hadrian, which makes him the next emperor. However, Hadrian never confronts her with his suspicions or demands an explanation, as he does with his guardian and mentor, Attianus, who orchestrates the assassinations that mark the start of Hadrian’s reign. Plotina remains a respected friend of Hadrian’s throughout her quiet widowhood.

Arrian of Nicomedia is another historical figure who plays a supporting role in the Memoirs, serving as a foil to the protagonist. Hadrian comes to identify Arrian as one of his few friends and considers him very nearly an equal, though Arrian is 12 years younger. Arrian is an accomplished military officer and political operator, and is also a philosopher who spends two years transcribing the philosopher Epictetus’s last words. From this, Arrian gained Stoic moralism and simplicity: Hadrian describes him as being neither self-righteous nor prone to excess.

Arrian is originally from Bithynia, where Antinous was born, and he marries a well-born Athenian woman. Hadrian compares Arrian to Xenophon, a respected scholar and philosopher, and a skilled military commander from ancient Greece. Hadrian’s admiration for Arrian reflects his admiration for all things Greek; he is also glad to have a man he can trust in a position of governance.