In his first life, Kunzite did not think of Mars much.
Of course, he knew her as one of Serenity’s guardians, and he occasionally spoke with her when she arrived to fetch her home, but the red soldier did not make as much of an impact on him as her orange counterpart. She was far closer to his expectation, and so did not stand out.
It was only after returning to life after being trapped in his stone that he began to pay attention to Sailor Mars.
Something about her that he appreciated was that there was far less baggage. Naturally, there was some ( he still destroyed her kingdom & was responsible for a great deal of atrocities, after all ), but to him, at least, there was nothing truly personal about it.
He found her shrine relaxing and calm, far better than the over-crowded chaos of the city itself, or the over-stimulating mess that was his shared apartment with his fellow kings. It was nice to have somewhere like that as a fall-back.
As for Mars— or Rei, as is probably more appropriate— herself, he finds her company pleasant. He finds himself not having to put up as much of an act around her as he does around some others, and while he is aware of the fact that she could literally roast him should he step out of line, he finds that this isn’t an issue if he simply acts like a decent person.