Kaido Yuji sometimes seems to hum a song (which said in the anime was a popular song before the "Blue" incident)
Yuji first hummed this "song" when he was first awoken by soldiers from second earth (the scene with a soldier who maintains his armored shrike) in Episode 3 or 4 I think.
Yuji said that this "song" he was humming reminds him of his best friend who had promised to see him after a "cure" has been found for his disease (episode 21)
Is this a real song, or is it just a fiction? Do anyone know the title of the song?