Well, Kirche in temperament.
The rule of mirror universe counterparts is they have to be able to be confused with the non-mirror universe counterpart with a little bit of disguise (usually a change in wardrobe, some light acting, and for the gentlemen a bit of a shave or fake facial hair), and trying to squeeze Kirches temperament into a small, bluenette frame... Her temper probably runs hotter. Which is appropriate, given blue flames are hotter than red ones.
That's also probably how Tabitha managed to sneak up on her - hothead was easily distracted, proving once again that evil prevails because good is dumb.
Mirror universe Tabitha was probably the Blue [Th/Bl]under of Tristain Magic Academy, and (this is just speculation) she probably idolized Karin for killing Nirak, which would be why she'd even try to enter the non-mirrorverse in the first place.