Sister Nia was a nun who was a passenger of the Lombardia and like others landed on the Isle of Seiren when the ship sank.
Adol and his team found her when she entered a dark cave trying to find mushrooms to eat.
Nia was unquestioning in her believe in God but her belief was shaken when the Nameless Ripper killed Captain Barbaros, as she truly believed that a good man like Barbaros should have been saved.
After seeing Adol's strong and pure will she became inspired and recognised that, while God loved all, it was up to humans themselves to shape their Fate.
Her belief stronger than ever, she put her whole being into helping the other people of Castaway Village however she could, like teaching Licht about medicine or cleaning the clothes.