Naozumi's past is similar to Sana's in that he grew up without his natural parents. But unlike Sana, he was never adopted and was raised at an orphanage (a pretty nice place where baby Sana also stayed for a few days). There, he learned about Sana through his foster family and by watching her on TV. Knowing she was like him gave him inspiration to be strong, and he followed in her footsteps to become an actor.
Naozumi gets to meet Sana when they shoot a commercial together ("heal ban? ....heal ban… heal ban? ...heal ban…"). After that, since he has a massive crush on her, he tries to get her affection by being her friend and helping her when he can. Because of his adoration for her, and noticing Hayama gets a lot more attention that he does, he sees him as a rival for Sana's love (not that Sana notices any of this, of course).
I used to really dislike Naozumi. In fact, he was the only Kodocha character I actually hated. I think it took me a long time to get over the way he acted the first time he showed up ("I know your secret."). That incident made him seen manipulative, and after that all the nice things he did for Sana always seemed more for his own benefit than hers. His initial attitude towards Hayama was also a factor in my opinion of Naozumi, but I think that he only acted that way because he felt very threatened by Hayama. He'd been doing his best to become a sort of overachiever to be like Sana and be liked by Sana, and then when he finally meets her he realizes that his rival for her attention is (in his eyes) just an unremarkable boy who's also his complete opposite.
But later in the series we get a different perspective on Naozumi. Even though he eventually learns that he can't become more than Sana's friend, he becomes her support and true friend. After that point, it becomes pretty obvious that Naozumi's feelings for Sana are sincere and selfless, and that Naozumi is in truth a very nice person.
For a long time, we are shown Naozumi's life as pretty close to perfect, since he has his large foster family to support him and a great deal of success in his acting career. Yet when his birth parents come into the picture and his world almost crumbles around him, Naozumi seems to come out of the experience a better person. Maybe it's because Naozumi hadn't really been that close with anyone until he shared that experience with Sana, and going through it helped him get more in touch with others. That's probably why he later seems to act more like a normal kid and less like a polite and distant "child star."
