Joss is the father of a terminally ill child.
Joss brings his daughter, Ghita, to the House of Black and White, after various treatment attempts have failed. Arya Stark puts his girl out of her misery by giving her the poisoned water from the pool in the central sanctuary.
In the books
In the A Song of Ice and Fire novels, there are several characters named Joss. The only one who is a resident of Braavos is a mummer from "The Ship" playhouse, dubbed "Joss the Gloom". However, many people go to the House of Black and White to seek and end to their misery or that of their loved ones.