Who is Fra Antun, Aldo from The Tiger's Wife: A Novel and what is their importance?

Fra Antun is described as a young, agile, outspoken, and compassionate Christian monk. He runs an orphanage in an abandoned monastery, where Natalia and Zora work with him to inoculate children against disease. His comments about the death of his younger brother, Aldo, provide an important clue in the mystery surrounding the identity of the man at the crossroads.