Having just read Hisako’s backstory, but not the manga, this was a really cool discussion to stumble upon. This is what I learned from 1st beat.
Hisako was in a band called 汚れた血(means like “corrupted blood”). The vocalist of this band called himself Kirisaki, though it wasn’t his real name. That is what the quote would be referring to.
The rest of her backstory gets a bit grim. Kirisaki was extremely suicidal, and Hisako had the responsibility of keeping him alive forced onto her. Kirisaki had a lot of fans, and due to their seeming close relationship, those fans started spreading rumors and hated on Hisako. After coming to one of her concerts, Hisako’s very supportive parents tell her that she should stop and come home. Hisako however realizes she can’t because Kirisaki would kill himself. Close to the release of the band’s second major CD, the harassment gets to the point where some idiot burns down Hisako’s parets’ house, and this is what makes her quit. After quitting she watches a TV-interview with Kirisaki about the new release and stuff, and in the middle of it Kirisaki pulls out a gun and shoots himself in the head. So, Hisako is basically suffering from the guilt of leaving Kirisaki. After Iwasawa disappears, she tells Otonashi that she had constantly been seeing Kirisaki’s ghost behind Iwasawa when she performed. Hisako was also mentally unstable, to such a level that she was confined to a hospital room.
Hisako’s route can be so unbelievably awesome. I hope it’s not a romantic one because that would feel cheesy I think. They have the setup for exploring so many other parts of how she would regain the will to live, so it would be a shame to have a normal romance like the ones we’ve seen so many times.