Japheth is the younger of two prayerboys who accompany The Prayermaster in his traipsing around Loggia City. The three of them later turn up in a helicopter to kill Romanov on his island. There the two boys commit a murder and escape in the helicopter, but turn up much later in the book, to no good end.
Japheth is described as fair, with a pointed face that turns mauvish when he gets angry. He pinches people who are in a weaker position than himself, and twists the pinches. When Nick Mallory laughs at him on Romanov's island, Japheth is humiliated and furious, telling him that now he REALLY hates him. A pious, manipulative rascal with no redeeming features.