Katherine started out evil as well.  Rita put her under a spell in order to spy on the Rangers.  Rita even turned her into a cat at times to make it even easier.  When Kimberly and Aisha found it, they named it PC.  They should've named it Kat the Cat.  Slowly but surely, Katherine fought the spell, turned against Rita, saved the world, and became a Ranger.  After a while, she and Tommy got closer together, and Tommy's broken heart was put back together.  So up yers, Kimberly!!