[He hates it on principle when someone tries to dictate anything about his life, ok???] Maybe there's a reason they don't want us to know.

[His expression turns a little thoughtful.] Have you noticed any patterns, in any of the other people who know? Like if anyone else is Earth, maybe they have things in common with your friend.

[a beat.] Unless there's a chance it's not related at all but - I've noticed that sort of thing lends itself to beliefs or traits. The book's probably a good start.