In English, 'who' and 'whom' tred tightropes between subject (who) and object (whom). It is wise to structure a sentence carefully. "You know it is who," or "...who it is," is a predicate nominative construction similar to "It is he," or "It is I," which are proper English but never used. Likewise, "It is him," or "It is me," are customary but improper English. Again, custom overwhelms grammar.