Both Itachi and Kisame were partners and they respected each other .
Even though Itachi was a spy , he din't seem to hate Kisame . When Itachi died , Kisame was really depressed about his death until Obito revealed himself to him . Before Kisame died , he thought of Itachi and their first meeting .
So where they good friends or where they just colleagues ?
Not at all, Kisame only respected Itachi's Power and thought they were similar because hey both killed their kinsmen. Kisame sees nothing in the world but lies.
You must be registered for see links
. Kisame said he "Understands" Itachi after saying he killed the Uchihas.
You must be registered for see links
Also the last memory Kisame had of Itachi showed that Kisame threated Itachi to watch out for him and it shows Itachi MS activated.
You must be registered for see links
and
You must be registered for see links
. This can be why Itachi Always had his Sharingan at 3 tomoes, because he didn't trust Kisame at all and Vice versa.