John Galietta III
I'm assuming you are talking about Mystique Unknown, which reads:
When Mystique is engaged, she copies the die stats and card abilities of one character that she is engaged with.
To define quickly, to be "Engaged with" another character, that character must be directly blocking your character or you must be blocking that character.
If your opponent attacks with Mystique and you block Mystique, they are engaged.
If your opponent attacks with Gobby and Mystique and you block Gobby, leaving Mystique unblocked, she is not engaged with any other characters and therefor can't use her power.
If your opponent attacks with Mystique and you block Mystique with more than one character, she is engaged with all of those characters and gets to choose ONE as the target of her power.
If you attack and your opponent blocks with Mystique, they are engaged and Mystique may copy your attacker.
Hope that helps.