Who is The Rabbi from Chapterhouse: Dune and what is their importance?

The Rabbi is a large, intense man wearing old-fashioned glasses, whose age cannot easily be determined, the Rabbi takes in the Reverend Mother Lucilla when her damaged spacecraft has to land on Gammu after escaping the destruction of Lampadas. Although she is not of the Chosen, he owes a debt to the Bene Gesserit for preserving Secret Israel during pogroms, so arranges for his spiritual child, Rebecca, a wild Reverend Mother, to Share memories with her before selling Lucilla to the Honored Matres to maintain credibility. The Rabbi and his people are rescued during the Battle of Gammu.