In my opinion Nariko has fantastic combo game and she is very versatile, she is high-mid tier at worst!
Fat Princess is really a great character as she has AMAZING approach, camping character will get torn a new one! Plus she is great for people who vrush her since she has plenty of attacks that would force someone who is approaching her to either go into the air, or stay on the ground, which gives her a HUGE advantage.
Spike may not be a powerhouse, but he has basic combos that can work on those that are both in the air, and on the ground. His specials are good, plus he has a couple trick moves to keep the enemy guessing. He is definently a good character.
Fat Princess is a great character, but I will have to say the worse might be... Actually they all look good to me and the beta no one felt overpowerd (Not even the Radec spammers) I don't think there will be a worse character since this game will be patched a lot. But if I had to take a guess, probably Sweet Tooth since his supers are easy to dodge.