That's one of the main points, and it's a good point. I believe that you're correct in saying Kobayashi intended to have her as the absolutely main character. but I think he abandoned that idea in favour of a wider cast. As for her development, Tenma has developed, only very slightly. For example, Kobayashi added a few talents for her, like her skiing thing (Was that in the manga? Can't remember). Then again the skiing thing was pretty stupid.
Originally Posted by reinard-fox
I suppose Tenma would be what people have been saying is a static character. That's not necessarily a bad thing. One way to look at it is that it not everybody matures at the same age, and Tenma is the embodiment of that. Compare that to, say, Yakumo, who is younger than Tenma but more mature. It's a personality thing. It would be unrealistic to say that everybody is developing at the same time when the cast is that wide. If you've got one main character then that character should be developed. It doesn't have to be that way when you have multiple main characters, as is the case here.
You make your face a mask. A mask to hide your face. A face to hide the pain. A pain that eats your heart. A heart nobody knows.