Who is Kostas and Daphne Mitsialis from Fugitive Pieces and what is their importance?

Kostas is a professor who shared an office with Athanasios before the war. Daphne is his wife and a painter. Jakob and Athanasios visit the couple when Jakob is thirteen, and he is amazed by their kindness. Kosta keeps in touch with Jakob throughout his life. He dies around 1970.