I haven't seen all the episodes with Kate in, but what I have seen, I didn't really take to her - sorry.
I prefer Ziva how she is now that she's developing a softer side to her character and is slowly being given more of a team role, rather than the all-seeing, all-dancing ninja-warrior-spy, she came across as at first!
She seems a lot more human now.
Not sure if Tony/Ziva are a love item waiting to happen or not, though - she has a real affection for him now, I'm sure, but I think it may be a sisterly kind of love.