what is with the missing chapters? does it effect how it is in order, like just skip a large part of stuff?
Just read it here: http://www.mangago.me/read-manga/sayonara_itoshi_no_my_friend/ . There is at first the story of his brother and then comes his story. But they are just lightly connected so it doesn't matter what you read first :)
I already love both of them. Not only for the art. I am 90% positive the only condition is that his cat likes the other person. I know this because that's how I feel. And the last words being that the cat liked the other really made it seem that way.