What exactly did you do to make Rinkah that good?
I just believed in her.
Really though, it's mainly that she has a nice range of classes that she doesn't need to marry/befriend certain characters to get. I just went through all of those classes as I went through the game and collected all of the skills so I have a nice range of skills to swap in and out. Still have three more skills to get from Dread Fighter itself and those ones are pretty good themself, especially Aggressor. Also she has Lancebreaker currently equipped and clubs beat lances as is so yeah she dominates anyone with a lance, no questions asked.
She still does really well against everything else though more or less as long as she equips the right weapon. That's really the only thing you still have to be careful about, you might be able to take some enemies out but if an equally leveled character with weapon advantage comes out that's still gonna mess you up. Though this is something very character has to deal with, and at least if I have a Replica out, the Replica can switch the weapon to a more advantageous one after making a move with Rinkah, and vice versa.
I gave her some permanent stat boosts earlier on too, but in general, she's just one of those "seems bad at first but you can make her awesome if you know what you're doing" characters.