I don't know if this helps, but I am able to get a similar error if I name my pikachu: 
It accepted the name, and the next time pikachu showed up, I got the following error:
I'm sorry, but an uncaught exception occurred.
While running game code:
File "game/prologue.rpy", line 134, in script
Exception: String u'' ends with an open format operation.
However, if I name Red "", it accepts the name, and no crashes occur. If I name them both that name, it crashes when pikachu comes on screen.
Maybe HailLordRed got the name to be blank by getting a tab character in there or something. Or maybe the input isn't sanitary, and there's a way from the input field to accidentally delete the pika_nick key from the dictionary.
Incidentally, nice touch changing "$" to be the pokeyen symbol in your name checker.