If you have the Defender trait, Hellcat is 1-ER to either attack or thwart 2.

That's a pretty fine deal. If this was a 0-cost event that just said "thwart 2," I'm sure that would be good in and of itself. Having the option to also use it for attacking, or pay a premium to put her fully into play if there are no other good options in your hand, is a nice bonus.

She's practically unplayable without you having the Defenders trait. The only reason I could see a non-defender using her is if they REALLY wanted to give themselves a Black Belt and was loading their deck with as many Martial Artists as possible. Otherwise, I see no value for non-Defenders in having her when she'd just be a strictly worse War Machine.