>>3868152
In the main story, you’ll be getting new monsters all the time and lots of the later ones are better than the early ones, so I wouldn’t worry about it too much. A girl will only get underlevelled if you don’t have them in one of your backup party slots. You can also catch up a girl’s levels to Ino by giving them a bit of mana each time (convert money into mana at the cat), it levels them up one level every time you do it. If you care about saving Ino’s virginities for (you) or whoever though, then you have to be careful about doing that.
After progressing the postgame a small bit (I think there’s only one mandatory boss to get to the chickener room equivalent), you can get to level 99 very quickly with anyone. That’ll let you use whoever you want for the entire postgame.