That's because you can assign your helper manually, which you probably did on accident.
Go into Units / Character Equip/Lock / choose a knight / click the "Assign assistant leader" button in the upper right corner.
If you choose an evolved knight you can toggle through her evolved and unevolved forms by clicking the "Exchange after evolution" button that's in the same place as the "Assign assistant leader" button previously.
The chosen girl will have a "Assistant leader" sign next to her Affection level. The sign will be red if the evolved form was chosen and blue for the unevolved form.
I'm not sure right now if you can return to the "first girl in the squad is the assitant" setting, but I think you can't.
So far I'm getting and keeping the time-limited reissued characters, because I've read early on that they have their use later. I'm not getting the permanent ones, since I don't have much inventory space.
I know now you can exchange gold+ units for a special currency that right now is not implemented in the Nutaku version, which in the end can even get you a "choose your favourite 6*" ticket.
Since we can't know if this will ever get ported to our version, I'm still deciding if I should keep them or make room in my inventory.
You can also level them if you want - they can be used simultanously as long as they're not in the same team, though this option somehow just doesn't sit well with me.