Who is Johannes Voelker from The Sparrow and what is their importance?

Father Johannes Voelker is Father General Vincenzo Giuliani's assistant. He is against Emilio Sandoz from the minute he arrives under their care in Rome. Voelker is very obnoxious and gets in constant arguments with John Candotti. In the end, Voelker has to humble himself when he realizes the severity of what Sandoz experienced on Rakhat.