ok, i am only half way with this but i am confused... why did kyousuke tried to kiss nina ...

i think kyou like nina, but he go out with asou because of sympathy

Maybe, just my thought

Though this is a one-year-old question, I still would like to contribute my two cents.

In one of the chapters where Kyousuke tells about what he wanted in a girlfriend, we realize that he didn't accept Nina's confession because indeed he thought of her as a sibling at the time. I think that he attempted to kiss her in order to see what kind of feelings her reaction will bring in him. Will it feel like a romantic kiss or will he still think of her as a sibling?

The kiss was never consumed because Nina already knew about Asou, so she didn't let him kiss her (how could she especially that he hasn't accepted her confession?)
Kyousuke initially thought of Nina as nothing more than a sibling and that he will never have with her what he wants to have with a girlfriend, something that he hasn't experienced before, something which will take him aback (and that was his biggest mistake).

As the story unfolds, he begun to witness with his own eyes those aspects of Nina's personality and reactions he hasn't seen before:

#1) her determination when she wanted something really badly like learning baseball - after that he caught himself trying to kiss her when she was asleep, but sadly again he fell in denial and told her in front of Asou and the others that she is important to him as a sister.

#2) Unfortunately for him, all this logic crumbles down when he saw Nina's flustered face for the first time - after kissing Miyake -. This is the incident which turned the tables on him and made him helplessly sad and jealous (though he tried not to make it obvious), but what could he have done after telling her more than once that she was like a sibling?
Ironically (and sadly), it is those words which made Nina lose hope and led her to discover the possibilities of a relationship with Miyake.

Nevertheless, the ending was a good one. I loved all the main characters though I felt sad for Kyousuke ...