I'm not the dev so I can't be certain, but it's possible that Raph is one of the names reserved for an upcoming character. I know not all characters in development are in the current build.
I thought that too, but then I tried naming my character both "Rohan" and "Anthony" and it allowed both, so I assume that isn't the reason it doesn't allow "Raph".
On another note, not allowing us to pick names of already existing characters would be a smart mechanic to implement (in my opinion).
