Short answers, pulled from the story.
Sidonius Apollinaris was a fifth-century Gallo-Roman poet, diplomat, and bishop, born around 430 at Lugdunum (modern Lyon). He is described by scholar Eric Goldberg as the single most important surviving author from fifth-century Gaul, leaving behind nine books of letters and twenty-four surviving poems that document Roman life and politics as the Western Empire collapsed.
Sidonius Apollinaris served as Urban Prefect of Rome from 468 to 469, appointed by the emperor Anthemius. He was also made Patrician and Senator by Anthemius, and held the title of comes after writing a panegyric for the emperor Majorian. In 469 he became Bishop of Clermont, a post he held until his death in the 480s.
From 473 to 475, Sidonius and his brother-in-law Ecdicus led the annual defense of Clermont against attacks by the Visigothic army under king Euric. When the city fell, Sidonius was imprisoned and exiled to Liviana, but was allowed to return and resume his bishopric in 476 or 477 after an intervention by Euric's advisor Leo.
A collection of twenty-four Carmina (poems) and nine books of letters survive, totaling 147 documents addressed to 117 different individuals. His letters were published in stages, with the final version appearing around 477, dedicated to his friend Constantius, a priest in Lyon.
A letter from Sidonius addressed to Riothamus, described as King of the Brittones and dated to around 470, provides evidence of a king or military leader with ties to Britain living near the time frame associated with the legend of King Arthur. It is among the most discussed documents in his surviving correspondence.
Sidonius Apollinaris is venerated as a saint in the Catholic Church, the Orthodox Church, and the True Orthodox Church, with his feast day on the 21st of August. After his death, he was first honored in Aremorica, the region covering parts of modern Brittany and Normandy.