Joaquim appears in both the long and short versions of “Of Ambergris, Blood, and Brandy” although in the novelette version he isn’t named and doesn’t actually get any face time. He’s Duilio’s cousin, more or less, a police inspector in the Golden City best known for finding people who are lost, especially cases other inspectors have given up for hopeless. Despite being related to a disgustingly wealthy family, he insists on living on his own pay, which means that compared to Duilio he’s always a bit threadbare.
As a character, Joaquim has a lot in common with Sirien (from Saturday). Sirien had no formal education, though, while Joaquim went to seminary (although he chose not to enter the priesthood in the end), but both are avid readers. Both characters are in serious denial about their abilities and neither one wants to face up to their parentage. Joaquim eventually accepts who he is, Sirien never does. Both live a monkish life (although for different reasons).
As for Joaquim, he becomes the primary character in “The Shores of Spain”….where he’s helping Duilio track down a lost boy.