Ok. There are some locked chests in the dungeon you might want to get into, here is the way around that.
Ok you want to play 6 of your own characters in the game. If you don't have a thief and really want to get into all the chests, simply keep one character out of the party until you finish the dungeon. SO you will have 5 of your own and Immy.
Then when you are done with Immy, simply remover her, save, exit the game, and re-enter the game making sure you bring your 6th character in at this point. Thus you will be where you saved, plus your 6th character will be with you. Your 6th character will be a tad behind the others in experience, but not that much, and you'll max out easily in the game anyway.
Thus you'll have a thief (Immy) in the first dungeon who can get into any locked chests, and you will wind up with 6 of your own characters at anypoint you want to remove Immy. I always call her Immy he he.