Well, that’s a good thing, I like it when I get to use different party members, but only if they were leveling up with me. If they weren’t, then that’s just annoying.
Now you have made me curious which one is it, that you are saving for last. If I am not mistaken, you have already done Bat girl and Cat woman. So that covers all the “good” female characters, that leaves Harley Quinn or maybe Poison Ivy, but that’s just the characters I can think of on top of my head, could be some obscure character like Azrael too…