Jachanan Ben Kathryn is his name, which means John Son of Kathryn. Good luck finding him, I have searched extensively on the Internet including Ancestry.com. No dice! He does not “exist.” If you read the first chapters of the book it says that God commanded him not to seek a following or be in the public eye (he is a silent ripple, see ch. 12:11-12)
Other names he could go by:
John Hirsch (his father is named Karl Hirsch (see ch. 2)
Yochanan Ben Kathryn (Yochanan translates to John from Hebrew, and means “Yahweh is gracious”). For reference, the Hebrew name for John the Baptist is Yochanan haMatbil.
Any possible names I may be missing?