Hmm, I can hear another voice, but there's no one else credited anywhere on the CD or lyrics book for that song (like Kondou is credited for Happy Goody, and Hoshi Souichirou for the duet with Gakuto), and knowing Hiroki's vocal talents, I'm betting it's him after recording a backtrack for the CD, especially since it's also there in the karaoke version, where you can REALLY tell it's Hiroki (the voice is quite high, and very much him XD) if you want I can rip it for ya :D