I'm gessing it's super-strength and if I'm understanding season 1 correctly, she needed Jessica for that, until the finale of course. But what is her power exactly? She has multiple personalties that give her a unique power?

I take it you've just finished season 1? I don't want to spoil anything for you if you haven't seen "Four Months Ago" yet.

to me, niki just had Multiple Personality Disorder and super strength was her real power. so she never needed jessica.