Joseph Fesch (1763 – 1839), as noted by Hugo in the novel's very first chapter, was Napoleon's maternal uncle. The young Fesch quits the priesthood at the start of the Reign of Terror, but comes back to the clergy in 1800. In 1802, Napoleon appoints him at the diocese of Lyon where he is soon named archbishop and then cardinal, thanks again to the intercession of his powerful nephew.
See chapter 1.1.I and also commentary to chapter 1.1.VI.
See wikipedia article: Joseph Fesch.