What did Niko find so damn captivating about Kate?

Packie warned Niko that Kate doesn't put out.

I don't know maybe after ten dates or so she would invite Niko in, but I only went to about five dates with her before the end.

Kate and spoiled? That is ridiculous, all the McRearys came up broke and struggling.

I think what Niko liked about Kate was her personality, the only decent one of the McRearys and perhaps the most decent character in the game.

As everything decent in Niko died during the war and after Darko's betrayal, I think with Kate he attempted to revive his soul in some way.

Kate wasn't one much for small talk. She actually talks about things that somewhat matter, unlike Michelle, Carmen, Kiki and Alex. I'm no so sure Niko was captivated by Kate, though. He would be upset if any woman he was dating got offed like she did.

Every girl in the game you can date has their specific preferences for clothes and cars. They don't tell you what they are, but if you're wearing and driving the wrong ones, they'll criticize Niko. If you wear the right clothes (from Modo) and drive the right cars (I think she tends to like 70s muscle) Kate will tell Niko how awesome they are. She still won't screw him, though...

The clothes aspect is definitely a game mechanic. You have to wear stuff from Modo for her to comment nicely on them. Which is funny because you wear a Perseus suit at Roman's wedding and if you chose to get Revenge, Kate will be there and actually comment nicely on it. As far as the reason why Niko fell in love with her is because she still saw good in him despite his lifestyle, and with that, Niko felt a sense of hope and redemption.