Alright, for this girl, you can make her as powerful as you wish, BUT, there is one flaw with her.
She can't ever tell the truth.
But, one day she's brought into a room with someone who can tell if you're lying. If she lies, she fails a test.
How does she pass this test?
AFTER YEARS OF COMMENTS AND GUESSES AND HINTS, WE FINALLY HAVE A VICTOR.