A question about Legend episodes. Can someone explain some things?

the other agent (mike) told the man in the diner tht G was a cop to gain his trust, as if youll remember it then led to a meet and revealed the identity of 'liam'. I personally think rivkins role was to show the relationship ziva has with the rest of the team, such as Tony the protective big brother, and Gibbs the intimidating father. Rivkins role also leads to the ncis season finale which is in israel (though i won't say more just incase you havn't yet seen it :D )
hope this helped your confusion!