What do the mirrors do, exactly?
Well they are like Rage Bands except there is one for each character and they are inferior. They have only a 25% chance to counter based on what attack hits the character. Here is the description of the items in the game.
Uranian Mirror - Counters non-light attacks 25% of the time.
A mirror bearing the image of a light deity.
Usable by: Chrono
Pontic Mirror - Counters non-water attacks 25% of the time.
A mirror bearing the image of a water deity.
Usable by: Marle and Frog
Promethean Mirror - Counters non-fire attacks 25% of the time.
A mirror bearing the image of a fire deity.
Usable by: Lucca
Hadean Mirror - Counters non-shadow attacks 25% of the time.
A mirror bearing the image of a shadow deity.
Usably by: Magus
Aresian Mirror - Counters non-physical attacks 25% of the time.
A mirror bearing the image of a warrior deity.
Usable by: Robo and Ayla