I came across something interesting, I looked at the routes that Kodaka routes that wrote and almost all of them have something in common,

And that is Kamyuhn.

None of the routes he wrote has Kamyuhn in it, (with the exception of Eva route, but he was a co-writer and not the main writer) because of this, I don't think Kodaka had any hand in making Kamyuhn at all, and let his team make her when the other writers asked him for their routes.

This would explain why Kamyuhn was made late in development, as Kodaka never planned to make her into a actually character until much later.