Don't worry, if you use Mirror enough it'll probably go away.
My understanding is Mirror increases your agility, so since rapier is an agility based weapon it makes sense that your damage potential increased.
I just beat this game, and read up on the mechanics afterwards, so I could definitely be wrong. Good luck on your playthrough. --- RPGConquest --- http://allconsolerpgs.blogspot.com/ All console RPGs from end to end