Katherine is a great character, with a lot of potential, that gets crapped on by shippers just because Tommy had the nerve to ask her out after Kimberly broke his heart.
First of all, her introduction. One of the greatest arcs ever in the entire 20+ years of the show. Kat is just the new girl in town, being friendly to a random dude, and Rita comes along to brainwash her into being evil. However, unlike when it happened previously with Tommy, Katherine is the mole. She's there to spy on the Rangers and report back to Rita. But it's shown repeatedly that the spell won't stick, and Kat is so pure of heart that she breaks the spell completely to save the person that Rita sent her to destroy.
Because of this, Kimberly views her as a worthy successor of the Pink Ranger powers. However, Katherine does have lingering side effects from the spell that are later purged completely.
Once we get to Zeo, Kat is sort of put on the sidelines in favor of Red Ranger focus and the show's new creator's pet, Tanya. But that doesn't mean that she didn't have awesome moments. Katherine held her own with a twisted ankle, agreed to a duel [that she knew was a trap] because her friends were in danger, and grabbed a blaster pointed directly at her face, saving a brainwashed teammate with the power of friendship.
Then we have Turbo, and while her episode focuses were not all that great (we do NOT talk about the anorexia episode!), Katherine finally showed us just how gracefully bad ass she could be without super powers. When has your favorite ever done a scorpion kick without a stunt double?
And before you say "but she was always yelling for Tommy! She sucks!" No, she wasn't. Katherine was usually yelling AT Tommy because he was being stupid and/or reckless. Kat was like his guardian angel, always trying to keep him out of trouble and from ending up dead.