At the end of the game Eternal Sonata who is the girl singing opera while the ghost of Chopin plays the piana is she meant to be someone important to Chopin and what song is it that shes singing

The woman singing is the Countess Delphina Potocka, a real-life friend of Chopin's. She was present during his last days, and it is said that she was so beautiful that Chopin, in his delirium, mistook her for an angel and asked her to sing for him. They brought a piano to the door of his room and she preformed The Canticle to the Virgin (which was said to have once saved the life of Stradella), and - when he requested another song - hymn from Marcello. I'm not sure whether either of those songs is what's actually played in the ending of the game.