From JP's interview with Rolling Stone we can be sure that the following characters are portrayed by James, he specifically says how he suggested him to sing them:
I would say things like, "For Gabriel, I want you to sound like you're 25-year-old James LaBrie." Faythe is a female, so I wanted him to sound female. Arabelle is an older female, so: "I want you to sound wise. Nafaryus, give him some attitude and wrath, be a little snarky with him — he can have a little bit of an attitude." [...] Every once in a while he'd be singing ... maybe he sang something from Arhys, who's a rebel commander, and I would say, "James, if that's coming from him, I think you should sing it with a little bit more strength, because he's supposed to be a commander."