I had been putting off playing as Agitha because I was nervous I wouldn't like her (I'd heard she's a slower character). My gosh, she is powerful. Her attacks do have a little bit of an odd windup, but the range and area are so utterly broken. The very first mission I tried with her was the mission to get her second ranked weapon. I collected an "A" Rank, both the heart pieces, and the first Skulltula, and I still had plenty of time left over. She's ridiculous.

I really hated Ruto at first as well. She takes some getting used to, because you need to understand how to time her attacks. If you charge in blindly, you'll get struck out of 90% of her combo finishers, and lose a lot of health, because she's vulnerable during most of them. You need to manipulate spacing, and sometimes even start combos before you reach your enemy. But she winds up being really powerful, and probably the best area clearing character in the game.

BTW: When you used Fi, did you remember to power up Light Energy with a non-combo Strong Attack?