User blog comment:VeiRetning/What UR Cards are worthy for Mirror Maiden?/@comment-24076773-20170412102016

"Summon that is Exclusive and will not return(E.g. Nadeshiko, Baal, Fu Tsu)"

Honestly I don't think any card is safe from getting a reappearance, I wouldn't use the mirror for that reason only. Also with Goth Oracle in the witch gate is seems ranking cards might not be event exclusive anymore either, so I would scratch that one, too.

As a general rule of thumb I would say, if you get by well enough at the moment and can't decide what to use the Mirror Maiden on, safe her! Because I'm very sure there will be a point in the future when the right card comes along and you will think I really need this HUR/GUR right now, good thing I saved her up :)