Kyoko and Izumi are classmates, though they don't share a lot of time together up to the moment in which they find out that they have something in common: the fact that at school and at home they're two different persons. From that moment on, Izumi will start to help Kyoko with her houseworks (since her parents are not always at home, she's the one that takes care of the house), becoming a friend even with her little brother Souta. As a result, they manage to know better each other and the way they feel inside of them. Izumi has a lot of problems because of his past with his strange manners and appearance, but with Kyoko's help he will believe in himself a lot more (even if he's still working on it) and he will change himself in order to be someone who can stay together with her (since she's very famous at school and people couldn't believe that they were dating) cutting his hair *sigh*.
Even if they are akin in some ways, all the other ways can be very different. She's a more direct type that would do everything in order to reach her goal, instead, he's quieter, though there are people with whom he's a lot more himself. They both rely very much on their friends, even if Izumi is the one who usually talks more with them to have advices or suggestions.
It will come the moment in which Izumi will meet Kyoko's parents and even if I thought that they wouldn't accept him, it was the exact contrary: I think that after seeing him once, he was already Kyoko's husband for both her mother and father. So, he's aknowledged by everyone in that family.
They usually never fight, but when it happens, it means that the main problem is their jealousy. They are very possessive and there's not a single occasion that they never let one get away, fighting against the whole world to protect the one they love or just their relationship. Anyway, they ended up being a couple without even noticing, because I think that they just realized they loved each other even if they don't do what canonic couples usually do.
After becoming aware of his personality when he's angry, we even find out that Kyoko is a sadist and a masochist who tries to be abused by Izumi (even at school...). But he can't be that self when it is about her... or at least, not completely because he doesn't want to hurt her (though she does).